Crown PCC-170
 

I picked one up used on ebay, did a side by side comparison and found it had a significantly more noise than an Audio-Technica U841A I was borrowing. I was surprised since the PCC-170 is a more expensive mic. So was I unlucky to get a sub par mic or is the U841A simply better? Btw, I recorded a sample of each under the same conditions, mixer and gain, and I could hear a noticeably louder hiss on the PCC-170 during the quite parts when no one was speaking.

Look up the specs on your exact AT mic, some of their boundary mics are very high sensitivity and some are regular sensitivity. What that translates into in the real world is when comparing mics don't use the exact same gain settings but instead use the same test subject and set the gain for each mic to give the same recording level.
Then in silence compare the noise of both mics at their realistic gain settings.
The used Crown mic from ebay could have been subjected to high temperatures, smoky or humid environments or the cable connections could be oxidized.
Lots of variables, but I'd start first with making sure you're getting equal signal levels from each mic under real conditions on equal sized boundaries, then compare the noise in silence and start eliminating variables one at a time.

When I listened to the recording closer I noticed the PCC-170 captures the high frequencies which is where I'm hearing the hiss. I'm guessing that this is the problem. I tried turning the high freq. adjustment screw underneath but I can't hear any difference.

Here's a piece of the recording maybe you can tell me whats up by listening to it. Btw, first is the PCC-170 followed by the AT 841.
http://creationdream.com/download/pcc170_vs_AT841.mp3

Specs:
AT841
Frequency Response: 30Hz - 20kHz
Output Impedance: Phantom: 200 ohms
Signal-to-Noise Ratio: 73 dB, 1 kHz at 1 Pa

PCC-170 http://www.crownaudio.com/pdf/mics/126443.pdf
Frequency Response: 50Hz to 20kHz
Signal-to-Noise Ratio: 72dB at 94dB SPL
Output Impedance: 150 Ohms, balanced (recommended load impedance 1000 ohms or greater)

Do you have the newer model U841a or the older AT841a. The U model has the better signal to noise ratio of 73db, the older AT841a has a much lower specification (65db) that you can find in the discontinued products list on the AT site.
If you have the older AT841a then I would say something definitely is noisy about your PCC-170. If you have the newer U841a, that comparison is a little closer, but the PCC-170 does sound both noisier and it's picking up higher frequencies too.
Since the PCC-170 has much greater sensitivity than either model of the 841, I'd hook both mics up to the same mixer at the same time, pan them apart and adjust the mixer trim and gain to give equal useable output (not equal settings) using the same sound source.
Record the separated tracks to a camera or recorder, then capture them to a non-linear editor so you can listen to each track separately but pan-centered.
It's possible that once you compensate for the much higher sensitivity of the Crown, you'll reduce the noise also.
Is the HF adjustment on the Crown a regular potentiometer or something with very fine control like a 10-turn pot that may not be very sensitive?
Make sure your mixer is giving at least 12 volts of phantom to each mic, make sure the boundary is the same size, distance to target is same etc.
Since the AT is a half-omni and the Crown is a half-supercardioid, depending on the room you're in they may never sound very similar but you should be able to tell for sure about the Crown's noise.
If the Crown is still much noisier, then I'd ask for a return or just relegate it to high-noise, high-risk recording environments.

thx for the detailed answerI have the newer U model and it's clearly more sensitive than my pcc-170.
The test I posted, the mixer was set at equal levels (7), since the PCC-170 is less sensitive than the U841a, I would need to raise the levels even more making the PCC-170 even noisier. Doing the right/left channel setup would yield and even better comparison. I might redo the test but I'm sure I'd get the same results.

I'm using a SignVideo Eng-44 mixer which supplies 12-24v more than enough phantom power.

I bought it used from ebay from someone selling of a bunch of these from a corporate conference room. Unfortunately no returns on this item, I think I'm going resell it on ebay. It only cost me $80 so it was worth a chance. Even though it looks in good condition age could be playing a factor.

I think I'm going to buy a new U841a, even if a new pcc170 was more sensitive with lower noise its pick up pattern is too narrow for what I need, which is depo safety net to pick up audio from non mic'ed ppl, who could be located in any position in relation to the boundary mic. But I must say I did like the clarity of the highs with the PCC-170.

FWIW, I used a pair of the Crown PCC-160s on a grand piano a few years ago, where the lid "absolutely positively" had to be closed. With few options, I gaffer-taped the 160s to the underside of the lid, in a sort-of wide 'V' formation.. slightly towards the high-end, the results where astonishing. It sounded so good, I took a production photo for a reminder. The A/V belong to the client which I have no current access too. The mixer was a Midas. The SPLs inside a grand are significant so noise was never an issue.

UCC Synod 24, Minneaplis, MN

With the piano scenario you're right on top of the sound source and its volume is considerably higher than people talking in a board room who are 3-6 ft from the mic. In my situation, I'm using a good amount of gain which magnifies any extra noise either in the mic or the mixer. But like I said I do like the clarity of the pcc-170 and its pattern minimized sound reflections of the room. Maybe I got a bad copy.

Yes I think you got a bad or worn out mic, although it's possible the HF adjustment could be contributing to the character of the noise.
I also have a pair of PCC-160's and thought they were higher quality mics than the PCC-170. However when looking at the spec sheets just now they are listed as being identical except for the HF adjustment. So based on my PCC-160's which I have used in lower volume situations, as well as my various AT boundary layer mics, and a Crown PZM-30D, it doesn't sound like your mic is working well.
A few years ago I was in a similar situation. I bought a CAD boundary layer mic from a different auction site that came from a church installation. It too was much noisier than I had hoped but my original intention was to use it in high-risk, high-noise situations so it wasn't too much of a disappointment.
I think in many of these installed systems the mics are powered 24/7, sometimes for years, even when the PA amp is off.
A new U-series AT mic should also give you better resistance to cellphone interference.

I changed the HF adjustment screw, I could hear the highs reduced but the noise remained the same.

Most of the time used means some cosmetic blemishes but works the same as new. What you say makes sense, they must have left them running 24/7