I notice when I hook up my at899 via a xlr to miniplug that the output from the mic is alot lower than with the standard mic that comes with the sennheisser wireless kit. The at899 comes with it own battery power supply. Is there a special xlr to miniplug adaptor that could boost the signal or have a lower resistance or something that would improve the output?

I know that the sennheiser has no line or mic input switch but you just need to wire the miniplug up in a certain way to make it a mic or line input. I'm sure the guy I got it from wired it up right.

I think the standard mic that comes with that
wireless is on the hot side anyways so, in
comparison, the 899 ...

I set the sensitivity of the transmitter to zero decibels which is the highest it can go. I get reasonable levels I guess but if the sennheisser mic is hot than its O.K.

Is there special connectors that can cancel out the small amount of phantom power the transmitter gives out to run the sennheiser mics? Will defeating this improve the audio quality or doesnt it matter?

I also notice that the at899 seems to be supersensitive to high frequencies like things ending with "s" in it and that things like plastic bags crinkling seem to be picked up too well. The curve on the info sheet that came with the mic showed a flat response so I 'm curious if others have noticed this too. Can you do anything to attenuate this as all the "s" sound very unnatural.

I also notice that the at899 seems to be supersensitive to high frequencies like things ending with "s" in it and that things like plastic bags crinkling seem to be picked up too well. The curve on the info sheet that came with the mic showed a flat response so I 'm curious if others have noticed this too. Can you do anything to attenuate this as all the "s" sound very unnatural.

It's not so much a sensitivity as it is problematic placement. This mic does have a very small bump around 2.5KHz, but that's not what you're hearing.Try placing the mic elsewhere, such as in the sternum pit, or other non-direction to air movement, and have talent be aware of action in the proximity of the mic.
The AT is no where near as hot on output as the Sennheiser if memory serves, so it does indeed require more gain.
Turning off/reducing phantom power won't affect the quality of sound on the Sennheiser, no. And the 899 requires phantom, so you'd not have any audio without the phantom in transmitter.

The power from my at899 comes from either phantom or the battery so I use the battery when I hook it to the wireless.

I deal more with images than sound but I'd thought I'd dabble.Its been alot of fun comparing the wired lav vs wireless vs shotgun mics. I compared the at899 to the sanken cos11 which was twice the price. I was not sure whether the sanken had the bass roll off on or not when I was comparing it but the at899 showed more background noise but it had alot more presence like the person was there talking to you rather than being abit distant. It felt like the at899 had more bass.I'm going to check if the sanken had the bass roll off on but my impression is the sanken is more neutral and rejects unwanted noise better.

I wonder if this feature of the sanken is more desirable to have. Can you add bass in post to get more "presence"? I imagine it would be an ass to get rid of background noise.

I'm actually enjoying learning this. Thanks for your replies guys.

If you haven't already tried it, place the hard screen on the 899 and then the foam screen over that. This will attenuate the high's a little bit and increase the resistence to wind or breath noises too.