Beatlebum - At those db levels the V40 would be fine. Like I said you get a little more control as you move up the line. My speakers are Dynaudio C1 sig rated at 85db. If your speakers are more efficient that is a plus. The Octave with EL34 tubes have less power and is better with 6550's or KT88's. You can run the KT120 in the V70 but I am not sure about the V40. BTW you will not get any more power out of the KT120 but you will be able to push them harder. The plate voltage of the V70 and V80 is 540 where the V110 is switchable between 540 and 620. Not sure but I would guess the v40 has the same 540 plate voltage.
I highly recommend the black box mainly because it lowers the noise floor and gives a bit more headroom.
I highly recommend the black box mainly because it lowers the noise floor and gives a bit more headroom.