Look up Hansgrohe Universal ibox - first of all, you will get a thermostatic valve instead of a pressure balance valve, thermostatic doesn't impact the GPM (gallons per minute) that the pressure balance does. Here is some hansgrohe info. and only will require you to have trim for this universal box - no additional trim needed. There are various options for configuration but I'm looking on page 16 of the "installation instructions" part No 90111601 and I'm reading "modify the diverter to allow simultaneous use of both functions". This came with my Hansgrohe trim - part number 04231xx0. I'm obviously a bit of a newbie but hopefully this info helps you understand that what you want can be done. I would go with hansgrohe for any shower fixture because their fixtures are famous for minimizing any restriction on the flow. You of course always want the option of having the maximum flow. Good luck. Hopefullly the experts will kick in here with more/better details if you need it. I bought my universal Ibox from a plumbing showroom and the salesman was helpful in explaining my options and what I could have done. Not much more expensive than buying on line. Here is a link that might be useful: see page 16 of this installation guide for hansgrohe trim 4231 This post was edited by elphaba on Sat, Jul 6, 13 at 21:39

Sharron, it was faucets4cheap.com, oddly enough! I called them to report a problem with one part and talked to them about my trim options - they mentioned the restocking fee, although I might be able to wriggle my way out of it if I tried, I suppose. Their prices are so great that it's almost worth it to pay the restocking, though. The other question I have (which I forgot to mention in the first post): do I even need a volume control valve? We always use the shower on full power, so I'm not worried about controlling the flow. However, from what I understand, the thermostatic valve can't stop the water from flowing so you have to have a volume control just to turn it on and off. Correct? Argh!

I think every manufacturer makes a thermostatic shower valve, so you might want to decide what style you like and go from there. I like Hansgrohe, they aren't cheap but they are very reliable.

You probably can mix and match components from different vendors; once a pipe leaves a fixture, valve, etc., the next item in the piping doesn't care what was before it.