Having the SLP-98 directly coupled will give you a faster pre-amp with extended highs and tighter and deeper bass, less tubey sound. Tubes that I know work well in the SLP-98 are as follows: Order of preference.
TungSol black oval (round) plates, blackglass, oval mica (very expensive)
Sylvania 6SN7GT blackplate bottom getter 1950-52 "Badboys" (expensive but can be found for a decent price if patient)
6H8C Russian Military 6SN7GT metal base (well priced excellent performance)
Sylvania Tall Boy 6SN7GT (great value, great tube)
Sylvania VT-231 (for $100/pr, a no brainer)
Ken Rad 6SN7GT - VT-231 JAN (same as above)