Suggestions for a preamp with HT Passthrough

Classe in an interesting thought. I don't know much at all about PSE HL-1, though.

Any preamp can integrate with HT. I'm running a Shindo Aurieges, about as 'pure' as you can get and it's in the mix with my 2 channel and surround sound set up. You just need an empty jack on your preamp to run your pre/pro into, then level set the volume on your preamp to match your speakers in surround mode. When you switch over to HT, set the volume on the preamp to the level-set position and you're good to go.

The quality through your mains may be marginally diminished, but I haven't noticed nor feel like I'm missing anything

Classe CP-50 and CP-60 would seem to fit the bill.
The CP-50 would be in your price range. The CP-60 is very similar to the CP-50 except with an outboard power supply.

I haven't actually heard these but I use a Classe SSP-25 now and am pleased with it.

If you can find one a PSE HL-1 would work;I enjoy mine driving audio valve challanger 180 mono's;it has bypass, rca, balanced outs, and remote control as well;I have not seen any in a while though.