Let me start off by saying after owning an original TS-9, and then having it modded by Keeley, I can safely say that the emulation on the PODxt is not even close, to be perfectly honest. Not to say it's useless, it is handy in case you don't have an actual TS-9/808 to run in front of your PODxt because it does emulate the effect that the TS-9 creates...just not true to the real deal. But that can be said about any of their emulated sounds, including amplifiers and cabinets. I would say the truest models of anything in the PODxt are the mic models. Haha. The 5150II and Rectifier models aren't really close to the real thing, however when I had my PowerBall I could get extremely close results using the PowerBall model in the MetalShop pack...you could still tell the difference, but a lot of the qualities were there. Anyway, I'm getting off topic here.
Now that I've said that, the point behind using a TS or any other OD pedal in front of a hi-gain amp for metal stuff is to simply use it as a clean boost. This just makes the signal going to your amplifier's input a little hotter, theoretically making the tubes react a little differently. The point of that is to make the amp tighter, basically. TubeScreamers in general add a little warmth, basically just a midrange boost if you want to get as general as possible, and a little more grit to the sound.
If your home-made TS seems to not be working properly, I would check the schematics with your work and also have a voltage/multi-meter on hand to test connections. Also, I know this is silly, but is the battery in the pedal fresh? Sometimes it's the smallest and simplest things that are overlooked and cause problems.
To reference, using the same general settings that you have found on this forum, here are two clips. The first is my JSX without the Keeley modded TS-9 in front of it, and the second is with it.
Without TS-9
With TS-9
That's so you can hear what exactly the TS-9 is doing to the amplifier.
~e.a