Nice to meet you! I'm pretty new here, too.
Congrats on the IIB. I've had mine (60/100w, wood & wicker, GEQ, Reverb, Export Tranny) since it was born, in 1980. I'm coming to appreciate it even more, lately.
But to your questions:
1. How to date it: A little wine, some flowers... no. Check the underside of the chassis. Written in black marker, you'll find a the date when the chassis was completed. If for some reason it's not there, call Mesa and tell them the serial number; they'll check it against their records, although they have been known to give the wrong info. (Example: they said mine was just a regular Mark II, which implied it was a Mark IIA. But I have an effects send and return on the back of the amp, which, along with the serial number and date of birth, nails it as a IIB.)
2. I retubed with Mesa branded power tubes, and the amp sounded... a bit sterile. I picked up some very old Sylvannia STR-387 6L6s and it roared. Much more harmonically rich and smoother, too. But I have limited experience in tube rolling. Try the Tubes subforum for some really good advice.
3. Mine is not modded. Almost without exception, people speak very highly of the Effects Loop mod (done only by Mike Bendinelli - $300), which alters the gain staging, provides an extra volume control to balance the channels (or use as sort of an attenuator), and makes the loop "useable." (Although I ignored it for decades, my loop seems to work fine with my new delay pedal; will post a new thread on this later today.) You can tell if yours has already been modded by looking at the back panel. If you have a pot labelled "POST FX LEVEL" where normally the Slave would go, then Mike B has had his hands on it.
4. I like the sound of 100W going into the 4ohm speaker jack, but I go back and forth between 60W and 100W. YMMV. Fool around with different combinations.
When dialing it in, use your ears, not your eyes. But keep the bass low, or it can get flabby (if you have a GEQ, you can bring it back in later in the stack; if not, grab an EQ and put in in your chain). There's a lot of amazing tone in these things, and a lot of real crap, too. Takes a while to get the hang of it if you're not used to the early Marks.
Let us know how you like it. Good luck.
