Ozzy, FWIW, I assume you have the SF manual and know what tubes are recommended for this amp by SF, however if not, don't make any assumptions. I have both the SFS80 and SFM's 160's. KT88 sound and work fine in the SFS80 so I mistakenly put them into the 160's. Poof! Then I read the manual - it recommended KT90's and 6550's and recommended against the KT88's is being incompatible.
If the sound in the SF P2 changes as it did in the SFS 80 - the 6550's (SED's) provided a very even tone from solid bass thru the highs but was a bit coolish. In comparison SED KT88's warmed up the lower-midrange and gave a bit of sparkle and air in the upper mids/highs. Personally I preferred the KT88 in this amp.
If the sound in the SF P2 changes as it did in the SFS 80 - the 6550's (SED's) provided a very even tone from solid bass thru the highs but was a bit coolish. In comparison SED KT88's warmed up the lower-midrange and gave a bit of sparkle and air in the upper mids/highs. Personally I preferred the KT88 in this amp.