Hey,


For everyone who is interested, I will reply on my own question just for the one who struggles with the same question. 

I tried a Celestion Alnico Gold and a G12-65 in the combo, and these are my conclusions.

The Gold is very responsive and accurate. Also, it's very LOUD!!
It's based on the Blue so in combination with the EL84's, the amp will tend to sound a little Voxy. Sounds great with a Strat!
This speaker will deliver exactly what you'll put into it. So, since this amp has lots of mids and highs, it lacks a bit in the low end. I had to tame the highs and mids and leave the bass full open. In band situations this will be no problem çause a guitar expells in the mids and highs. The bass is tight, actually I think it will be even better when you use a closed cabinet.

But I like my sound a bit warmer. So after a couple of months I switched the Gold for a G12-65 (an old one, early 80's). This speaker is based on the greenback and Marshall used this speaker quite a lot in the 80's, and so it does sound a little more Marshallish. The amp stays very responsive. It's warmer than the Gold, and rolls of the highs just a bit more. The bright switch is more usefull now and I can leave the bass at about 3 oçlock. The mids are a little more compressed but not too much. Still sounds great with a strat!

I think it's just a matter of taste what speaker you'll like best. Both sound great, but each has its own qualities.

The open combo is excellent for pop and blues(rock) sounds. I still want to try this amp with a closed cabinet, I like the punchy bass of closed cabs. See what it will do to the bass frequencies of this amp...