Passive Bi-Amping-Amps With Level control

Foolish1,
Virtual Mode makes a very good passive attenuator. Go to www.virmode.com and check them out. You can contact them by phone or email, the phone number is on the site. Brad is a good guy and will set you up.
Good luck,
Rodge

Can't you just adjust the integrated amp's output to match whatever solid state is on bottom, then introduce a preamp (preferably passive) upstream of both amps to control the output from your source level?
Otherwise you could consider the adjustable EVS attenuators for the amp(s), see www.tweakaudio.com.

Carver Pro ZR range have level controls and also a low pass filter.

I talked with the folks at virmode a while ago. they were impressive, although I did not try the product.