I don't own a KSP8 myself, but I have auditioned it on several occasions. Unfortunately, all those occasions were NAMM or AES shows on headphones, and the background noise at those conventions is very high. So I don't feel I'm really qualified to give you an in depth comparison between
the KSP8 and a Lexicon reverb.
Reverb/multieffects boxes I own:
Lexicon PCM-81/91,
TC Electronic M3000, M2000,
Yamaha REV5. What I often do for reverb is layer the PCM-91 and M3000.
As far as reverb character, the PCM-91 is lush and rich, although it can be programmed to be much lighter. The M3000 doesn't have that lushness, but has a nice realism to it. I think I still prefer
the PCM-91, but both come really in handy.
From what I've heard myself and what I've read from others, the KSP8 reverb is somewhere in between the Lexicon and TC Electronic stuff. I've also auditioned the Rumour/Mangler at NAMM/AES shows and they sound excellent. I think they are going to prove to be a real value for the money kind of thing.
It really would be good if possible to wait for the new
Yamaha SPX2000 and try that out, as well.
On the lower budget end, I personally like
the TC Electronic M-One XL for reverb. Impressive for the money and a bit better than the M2000 reverb in my opinion. I'm referring to the M-One XL, not the original M-One. If you got a Lex PCM-91 and an M-One XL you'd have the bases covered nicely. Or add
a Kurzweil Rumour to the PCM-91.
My philosophy when I was upgrading my outboard was to have one *great* reverb, and a couple secondary reverbs that were very very good. I ended up with several great reverbs, actually! But the approach works, and it's better to have more than just one fine reverb.
So perhaps a Lexicon along with the SPX2000, or a Lex and the lower cost TC or Kurzweil. However you combone them is up to you. It is important that at least one be stellar though.
Hope some of this rambling helps.