Altec 417s are somewhat readily available on Reverb and its easiest to lay hands on a single speaker. I have a pair of them in an oversized 2x12 and they are a much better speaker for guitar than the other "hifi" speaker for Mesas - the EVM12L, which IMO gets a bit strident in a Mark series amp...
I believe the FU2 is dramatically more complex than the FU3 and I wouldn't feel confident about building one since I don't have a Quad preamp to test with unfortunately.
A IIb should have the loop on the back panel. The jack on the underside is for the EQ/Reverb switch, which is the 2-button TRS switch. The channel footswitch connects to the front of the amp.
New MKIV owner. Found a guy to make a replacement head cab since mine came in a rack kit when I got it. Couldn't resist the zebra tolex and had to make a new foot controller based on the FU-3 logic switching scheme to match the vibe.
This is a direct copy of the logic switching scheme from the original Mesa FU-3 foot controller for the MKIV in a much more pedalboard-friendly format than the original.
Functions exactly the same but takes a detachable 6-pin XLR cable (https://btpa.com/MIC-XLR/4-5-6-7-Pin-XLR/xlr6-sc-xx.html)...
I think this is more to do with their current backlog of orders than anything else. I tried to order a headshell a month ago and they said they were not taking any orders for replacement cabs for at least 90 days. They likely just don't have the capacity to do custom tolex/hardwood/etc... at the...