Audio Research SP8 MK II Sounds

I have owned an SP8 for some years and am very familiar with the C-20 Mac. There is no way the SP8 is forward or boxy. It is extremely airy, rich and musical. The C-20 just does not compare. My guess is the Krell may have too low of an input impedance and loads down the SP8. Many high dollar tube preamps, like the SP8, do not work well with all solid state amps. The Sp8 is one of the best preamps ARC has produced and it exudes listenability. Voices are really special and real sounding. The phono section which is a moving magnet and quite good.
One other thought is that 861 is a little to clinical and solid state sounding in it's stock form. The Great Northern Sound mods transform it, making it warmer, smoother and richer sounding.

Amaka -
I live in the Tampa/St. Petersburg area.

Where are you located?

I'm in West Palm Beach.

Audio Research and Krell are mis matched

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!