It's been a lot of years since I had a Model 950, so I may be forgetting something, but in analog bypass mode the stereo signals went to the left and right channels with only volume control. The sub was inactive, unless you wanted to use the 6CH input with a separate phono pre-amp (in which case you had analog bass management using an 80Hz crossover). I think the Model 1070/Model 970 was similar.

None have offered the RR2150's bass management flexibility for analog sources, but then again they are different products with different focuses - the surround processors focused on doing bass management in the digital domain first and foremost, with analog bass management provided as a perk for folks with multichannel analog sources. I don't know how practical it really is to build in the RR2150's analog bass management capabilities on a surround processor that is focused mainly on HDMI.

If you convert the analog source to digital, I'd expect that you would be able to apply any digital processing that you wanted to, including bass management, matrix surround processing, and room correction. It may seem counter-intuitive, but some would argue we're already getting away from an intuitive configuration when we pair a traditionally two-channel analog device like a turntable with a traditionally digital surround system like a home theater. We get to a point where you probably need to choose: here it "as it was intended" (analog pass thorugh, limited or no bass management), or apply some or all of the full suite of digital tools now available to you.

If you pick the former, I wonder how much content exists below the typical front channel -3dB point on most recordings that would make missing out on having the subwoofer really a problem? In vinyl's golden age, we didn't have subs and lots of "full range" speakers rolled off around 40Hz or so. Of course, if your record collection includes a lot of pipe organ recordings, the answer to my question could be "lots of content."