Romy - Not sure if you mean increase the load above 550ohms, which would give you less power, better damping and possibly less distortion (trade off between how much you need to push the tube and the flatter loadline, also I suspect there is distortion cancellation going on in the Melquiades which is more unpredictable).
However, this would slightly increase the gain and more "overdamping" of the 6c33, certainly lower Zout...
Is this what you mean?
Something else to consider, if you are running your 4 drivers in series parallel, then connecting 3 parallel pairs in series will give you 1.5x the original load. Well, strictly speaking, it will if your OPT has plenty of primary inductance - as I recalll, 7H, which is getting marginal, but anyway, at least at 40Hz and up, the 6C33 will then see a load of 800ohms... so if you are defintely going to go to 6 drivers, this will avoid having to get the OPT rewound.
OTOH, if you are of the "series connection of woofers is bad" school, get those OPTs rewound for a 1ohm load (6 woofs in parallel).
Btw, assuming identical woofers, there is no change in damping between series and parallel connection I reckon, contrary to the BS usually spouted.