Audio Research SP8 MK II Sounds

First thing, get a lot of hours on those tubes before you make a decision. I think EH tubes are harsh and unlistenable for at least, and I mean AT LEAST 50 hours. Then come back and maybe start tinkering.
Also, I'd hate to agree, but Augustusjohn might have something. Both of those pieces are fantastic in their own way, but they may not be the best match. I'll check that tonight.
Were you happy with the C20? Because if you were close to what you wanted with the C20, I'd say you're on the right track with the sp8. I just love the ecc88 difference, it just blooms! What cables and XLR adapters are you using with it? I lean towards silver with the sp8.
Myself, I'd be deathly afraid of blowing amp/speakers up with the ARC's questionable mute delay!!!! I know, I've done it.
Also, I'll agree that the tubes make the biggest difference with the preamp. I'm using telefunken ecc83's and ecc88's. And it does not sound boxy in the slightest. I'll hook mine up to my fpb600 tonight to see if I get anything like what you are talking about. I usually use the sp8 in my analog/phongraph system with a d75a amp and silverline sonatas and rel strata sub, synergistic connections.
The krell is with reference one sometimes, but mostly directly from the dac/computer to wharfedale opus 2's which are a little similar to the nautilus design, except for the dome/cone midrange, which I think makes the nautilus a little dry and woody compared.
Whatever you do, Don't be like me, and make a rash decision until you give it some time. And get rid of those EH tubes!

I am an authorized repair station for ARC and a past owner of an SP-6B. I have also owned a McIntosh C-20. They are very different sounding preamps. The C-20 uses a tube rectifier and so the bass can be a bit soft. The top end of the C-20 can also be a bit soft due to circuits like tone controls and a limited frequency response etc. It does not have a flat frequency response. The SP-8 uses a tube regulator in its power supply. It has a much better regulated power supply in it then the C-20 which has virtually no regulation at all. It has a flat frequency response. Both preamps high level stages are similar. The phone stage is a bit different. The SP8 should sound more open, have better bass and have better top end, as well as lower noise. It's possible the preamp needs to be recapped and your regulated voltages are wrong. There's no need to update this unit, which I have done in the past, without first checking to see if the power supplies are running properly.

Tried it for a couple of days with the fpb600. Not the best, but not bad either. But I was wasting the amp with the single ended input. I think a little darker than it should be.
After thinking about it. Why did you get the sp8 (or even the mcintosh) if you're not using a turntable with it?
Strange pick of preamps, considering the rest of your set-up. I would think something more modern would work better with your rig. Good luck finding it though...