The two octal sockets on either side of the RCAs is a power socket for old school preamps that pulled DC right from the AMP instead of having thier own transformers and rectifiers. I have a couple PAM-1 (mono version of the PAS) which have the octal hookups but i'm a little weary of hooking em up like that, though they're supposed to sound very good.
Its quite a weirdo setup to have the inputs in the front like that, but i suppose it makes logical sense (Front input through to the outputs in the back).
Mullards are nice, If you happen to see a set around for a reasonable price jump on them... they made tubes for a ton of different brand names. Look for a hole in the guide pin and "made in great britian" on the glass somewhere. I like the SED tubes as well, they sound very similar to the mullards and are well crafted. I also tried a few quads of the big bottle EHs which have a slightly different sound, shall we call it darker?? but still very nice. I have a quad of japanese (mullard set up the plant over there) EL34s that I just don't like, I haven't really made an attempt to break them in much but i just can't put my finger on what I don't like. I still want to get my hand on some NOS telefunkens, I hear mixed reviews, but its tough to tell exactly what you get until you hear them.
At any rate, I think tube audio can be just as big of a money pit as taping... I love it!