In search of a 5.1 receiver with THX??

No Frills or Bells and Whistles that might work could be NAD T7#3 series, Arcam AVR200, amoung others and B&K makes a AVR-507 that bcosts a bit more.

Who cares about the THX lable? All it means is that they paid extra for that designation. Some very nice units don't say THX but will put out more clean power than many that are THX or THX II labled.
You should be more concerned about speaker efficiency vs. wattage. A 100 watt per channel receiver with 91db speakers will be way louder than a 150 watt per channel receiver with 86db speakers.

Check out the reviews for the Denon 3802 or 3803 on audioreview.com or audioasylum.com. I use the first for HT with M&K speakers and couldn't be happier with it.

There are tons of them out there. Any limiting budget or the sky is the limit? 2-channel music? FYI only, Yamaha recently released RX-V1400 for MSRP $800, which can be found for substantially less. It meets most of your requirements-- THX, Digitial Dolby IIx, 7.1 channel, work as pre/proc with external amp, and many more. It can do video up conversion, which is a nice feature for wife and kids since they don't have to switch inputs on TV. It does NOT have dual ports for dual subs, but many folks have used Y splitter. Not sure if you like such idea. I use RX-V1400 as an example under the assumption that you want some thing in such price range. Some will say don't spend the extra $ for THX. Some will say Denon is better than Yammie in such price range. It's all about personal preference. Good luck.