60W tube watts should really be plenty. Indeed the tubes may be dying, and with such old tubes I'd definitely change them already rather than risk a catastrophic tube blowout, but it might simply be that the 4x12" yields so much more dB at lower frequencies than your single - not to mention open-back - does that it seems like it's not enough. Closed-back and multiple speakers pump air so much more efficiently. It doesn't need that much extra to sound way overbearing in comparison with a tinny open-back single speaker.
Speaker efficiency after all is way more important for volume than actual wattage. Some 36W like my old Ceria was is quite enough to play along with 100W Marshall rigs when used with a suitable 4x12" with V30's etc. in it that are pretty efficient, but it could hardly be used with a drummer using another 2x12" cab I had.
Also, EQ and gain etc. settings can have a huge impact on volume. Despite I use a 100W tube Marshall head now, my 4x12" isn't the most effective plus my EQ/Gain settings are pretty low, I find that I have to turn the amp past halfway mark to be well heard! I really need to push the lead master volumes to near dimed to really be able to scream over the rest of the band.
So it might also be that you're far from 'unity gain' within the amp structure, i.e. at some point some stage is not feeding full signal to the next stage due to low gain setting or whatever, but I'd first venture it's the cab/speaker inefficiency that is to blame here.
And for devil's sake change those t00bs already :-)
Dee
"When life's a biatch, be a horny dog"
Amps: Marshall JVM 410H w/ Plexi Cap mod, Choke Mod & Negative Feedback Removal mod, 4x12", Behringer GMX110, Amplitube 3/StealthPedal
Half a dozen custom built/bastardized guitars all with EMG's, mostly 85's, Ibanez Artwood acoustic & Yamaha SGR bass, Epiphone Prophecy SG, Vox Wah, Pitchblack tuner plus assorted pedals, rack gear etc. for home studio use.