SQ are select toobs, supposed to be lower noise, blah blah blah, but I don't like most of them all that much and they command a huge premium.
Go over to Audio Asylum and look up joe's tube lore as there is a wealth of info. I have a pair and they are nice sounding tubes, not as good as my E188cc Amperexs but better than the Buggle boys or Mullards I have, so I guess someware in the middle and much better than the new stuff, at least the one's I have. I bought three of these and I think I paid around $50 and they were advertised as used but with plenty of life in them and came from a good seller. If your just getting around to tube rolling I would stick with buying from Audiogon rather than Ebay unless you know what your looking for and or know the seller, but pay attenion where ever you buy though. In a earlier thread you mention SF line 2 and the best position is LV2 and V2 per Chris Johnson.
As with cables, tubes are a completely subjective matter with many variable parameters ultimately determining their performance in any given application. Some people like the Philips E88CC SQ, others don't. Personally, I think they are on par with some of the finest E88CC variants around. Philips tubes literally are Amperex tubes (or in the very least they were manufactured at the same facilities). The earlier large print variety of the Philips is better, IMHO, than the later small type versions. Good listening.