Just checking back to see how things went: wow! a classic, swerving all over the road, Talkbass thread, with digressions in to sorta-scientific mumbo jumbo! I love it!
Meanwhile, back at the ranch, I picked up a Yamaha MG102c mixer off a scratch and dent sale for under $80. Looked like it had been resealed at the retailers. I had a cold and was bored so I unpacked it and reworked my practice station. Works flawlessly, and sounds great.
I run my bass though my floor stuff (MXR-M80 DI +) into one of the two channels with built in compression. (XLR) The Cd player runs through standard stereo RCA into channel 9,10; ipod the same into 7,8. Drum machine into the 2tr. Stereo outs to monitor and mains. Alesis microverb on the aux send and return. I can insert other effects into CH1 & 2.
Headphones get a stereo mix from the monitor or mains. Sounds complicated, but it's all together cleaner and quicker to use than the Rube Goldberg contraption I had going with the old compressor, Bass Rockman, and a mini stereo cable that I moved back and forth from one source to another.
Everything flips on with a powerstrip.
First thing I noticed was some of the training materials I work with have a clever stereo mix on the backing practice tracks: bass and drums only on left, everything center, and drums and piano right. Very cool! I couldn't do that with the old set up. No more moving the cable around, I just turn on the source and it's there.
I have high hopes for using this to run multiple inputs into my looper with better input levels. I still have room for a synth, guitar, and vocal input. With powered monitors and powered mains you could mix a small band.
So, to the OP's original headphone practice problem, I'd go with the small mixer. And I have the ipod deal and a couple other options. I've been working on this same problem for a long time.
I've blown $80 on lots less useful stuff, this thing is a winner.