Hey Sirion, the idea of headroom in the amp(s) compared to speaker power handling is more suited to PA AND you need to know what you are doing (and generally you have a compressor set to peak limit before the amps). Comparatively, distorted guitar is very compressed(ish) and better (IMHO) to have a bit more speaker capacity than amp, not that you can't do it, just have to be careful not to pop speakers
. It doesn't help that the 9040 has no vol knobs... So you choices are to:
control it with speaker impedance and/and numbers of (25w) speakers
use a compressor/peak limiter before the amp
use a power soak between amp and speakers (I have a Tom Shultz power soak I use with my 73 Marshall 50, not for the speakers though, just so I can be in the same room
)
To do it with speakers, depends how many 25w greenbacks you have per channel and their impedance. IIRC the Marshall can deliver 120w p/ch into 4 ohms but will be less into 8 or less again into 16 ohms.
So Ideally give it a 8 ohm load with 4 x 25s, nice and safe (but that's 2 quad boxes)
or 16 ohm load, and 2 x 25 w greeenies (p/ch) and be a "bit" careful how high you turn up the input to the amp.
It will also be ok at 8 ohms, but you need to be even more careful