How about a two-channel EQ similar to the Pioneer SG-50M here:

http://www.oaktreevintage.com/Stereo_EQ'S_And_Signal_Processors.htm

It is really hard to find a good EQ that can take advantage of the processor loop provided by the 2150.

That's an interesting suggestion. It was common for stereo gear made in the 70s and 80s to embrace the use of tone controls and equalizers. If you read the manuals of that gear the equalizer was to help correct room issues. I don't see why today's stereos like the RR-2150 wouldn't benefit too. Funny how we are back to that concept, albeit with home theater and a bit more sophisticated with things like Trinnov in the 998. I like equalizers and have a Pioneer SG-9500 that I use with other Pioneer gear made in 1977. Vintage EQs show up on ebay all the time and can be had for a few bucks. Given one a try and see if you like it; its fun for a small cost. At the very least all those slide switches look really cool!