I still own the Classe SSP 800 paired with all Classe Delta series class AB amps with all new B&W 800 D3 series speaker sysyem in my dedicated HT room.
I love the SSP 800 sounded phenomenal for both surrounds (home theater) and stereo. But I use my SSP 800 strictly for home theater (surround sound movies) only since this setup is strictly for HT movies use as I do have a dedicated separate stereo setup / system in a separate room. But I’ve tried my SSP 800 for stereo music playbacks used as a stereo preamp and it sounded really good especially coming from an AV processor. IMO the SSP 800 can also serve as a high end stereo analog preamp as well as surround processor.
It's stereo music playbacks isn't compromised as it's usually been the case with many AV preamp processors. But the SSP 800 is somewhat obsolete as it cannot decode the new latest surround sound formats eg Dolby Atmos, DTS-X, Auro 3D, cannot pass through 4k video signal, etc. The SSP 800 however can decode DTS-HD Master Audio & Dolby True-HD.
I’ve also heard the Classe Sigma SSP paired with its matching Sigma Amp5 and sounded fabulous especially in stereo music playbacks. But I have never compared both the Sigma SSP & the SSP 800 in the same setup and same system so I don’t want to come up with any assumption which one sounds better for which application or use, whether stereo or multi-channel movie (home theater) playbacks. But imo the SSP 800 would be superior for surround or multi-channel playbacks and not sure which one will be superior for stereo playbacks.
Both the Sigma SSP & the SSP 800 are excellent sounding preamp processor indeed and both of their stereo music playbacks aren't compromised.
In fact, the Sigma SSP is optimized for stereo playbacks for use as a high end stereo preamp.
But according to my audiophile buddy who happens to be a reviewer for SoundstageHifi.com and SounstageUltra.com, he told me that the Sigma SSP sounded slightly better than the SSP 800 in stereo playbacks or when used as an analog stereo preamp whereas the SSP 800 is superior in multi-channel playbacks (surround sound movies).
There are handful of features found in the SSP 800 that are eliminated or not found in the Sigma SSP such as multi-channel analog audio 7.1 inputs and legacy analog video connections, and while the SSP 800 has XLR balanced analog audio outputs for all channels including subwoofer channel(s) the Sigma SSP is limited to only front L/R channels with the rest of the channels are single-ended but optimized single-ended ones.
However, the Sigma SSP is more future proof, it is DNLA equipped and has PC USB assycrhinious DAC inputs which the SSP 800 lacks.
The SSP 800 has been discontinued last year and can be had at massive discounts nowadays. The original msrp of the SSP 800 was $9500 but today it can be had for around $3500 used if you can find a used one here on Audiogon.
Often times I spotted used SSP 800 processors here on Audiogon.
All being said, I would recommend getting the Classe Sigma SSP over the Krell Foundation pre pro. The Classe would sound better than the Krell Foundation imo. Krell as a company isn’t the same anymore since Dan D’Agostino’s departure from the company in 2009.