Mac MC240

1. Hi/Lo voltage 115/117 switch
2. Mullard 12AX7/ Amperex 12AU7/ RCA 12BH7 bk plate/ US brand 7027A instead of 6L6GC not Russian tubes
3. Home brew pure silver cable/ NBS Master speaker cable

We have 4 MC240s among our friends. The bugle boy nor Amperex (both made in Holland) makes no difference. If you want the best 12AU7s then go with Amperex 7316 tubes.
Cleaner without loosing the nice sounding. Even the Telefunken 12AU7s are nice too.
Reason why we didn't like the RCA 6L6GC bk plate vs say RCA or GE 7027A is the lower freq control. The RCA 6L6GC bk plate sounded very loose and lack of control for lower frequency response. The 7027As are much tighter and more dynamic and it is not thin sounding.
Yes, I'd designed and made my own silver cable after went threw over a dozen speaker cables and 50 interconnect RCA cables.
Just a hobbiest not a businessman.

Hope this helps.

Yes the 7027A are direct replacement in the case of MC240.
Telefunken 12AU7s are more smooth and refined but less dynamic and punchy when compared to Amperex 12AU7s.

The stereo vintage Mac amps are much more desirable. Both transformer's winding had to have same number of turns in order to make the match pair.
Since I don't have MC40 ( only heard the MC30s), I can't tell you how it sound. It is older than the MC240s. Aside from that, all vintage gear's condition can vary a lot depends on how much life left in the capacitor and the accuracy of the old carbon composition resistors.

There is a big difference between the Russian/China 6L6GCs vs the NOS 7027a and RCA bk plate 6L6GCs. The clear top RCA 12Au7 sounded warm and muddy. The top end roll off so much and the bass is merky. The GE grey plate 12BH7 also lack of clarity and fullness of the RCA bk plate 12BH7.
Definitely prefer the mullar 12AX7 long plate over everything else.
Again, just our observation in our 4 totally different systems with same results. Speakers range from Rogers LS3/5A 15 ohm, Proc 2.5, B&W 805s, ATC SCM10, platnum solos, Joseph audio RM-22si, Paragon regents... Preamp tested was ARC SP-11 MKII (3 different units) Counterpoint SA5000 (3 different units) and 2 different Marantz 7 preamps.

Again, opinions are subjective to our setups.

1. For the output tubes, the one in front of the left transformer is left channel ( The center transformer is power transformer) the vise versa. V8 and V9 are left and V10 and V11 are right.
Since MC240 are auto-bias, it is recommend that match quad is used. If not match quad then match pair should be 2nd choice. Honestly, I couldn't tell the difference even when tubes are not matched.

2. The first 12AX7 is the input tube for both channels.