They were serviced and recapped at the same time so they sound great together and can definitely be used on stereo material. They are racked and powered by a nice external BAE PSU. They run on +24 and +48V similar to Neve modules. The fader caps are not original.The channels include: - An 1108 preamp- Hi Inductor Eq with +- 10dB at 2.5, 5, 7, 10 kHz- Lo Inductor Eq with +- 10dB at 50, 70, 100, 400 Hz- Optical limiter- Overdrive / Gain Reduction indicator lamp (very cool and unique feature!)The preamp gain structure is set with a 0, 20, 40, 60 dB input pad and the fader controls the channel output. The indicator can be switched to monitor channel overdrive or the presence of compressor gain reduction. Mark Lawson who records the Arcade Fire describes how he used them to record allot of The Suburbs record below:"It’s a UA 2100 sidecar. I believe it came from Western or United Recorders in LA. I used the sidecar to record a variety of things. Each channel has a 1108 preamp, a limited EQ — broad stroke treble and bass — section, a very slow optical limiter and a large square metal fader. Every time the lights would flash on the limiters, I would smile and hope for the best. It was tricky to monitor sometimes in the makeshift DYI spaces. The title track, “The Suburbs”, with the dueling drum kits, was recorded with that sidecar.This is a really special pair as these hardly ever come up for sale. I can include a pdf copy of the manual for the buyer. Let me know if you have any questions and check out my other pro audio items.Thanks.