Two amps can have wildly different amounts of gain, so my first though is to have a way to adjust the levels going to each amp. If one amp has 26db of gain and another has 29db, that's a 3db difference between your low end and your mid/high. You'll need a way to compensate for that, unless you can get amps that are fairly matched
Also, you have a tube pre already, right? Are you sure you want a tube amp, too? I'm not trying to talk you out of it at all, just mentioning that you already have tubes in the chain (though you already know that).
If you do, you're looking at buying a tube amp right now. One option would be to sell the MF-80 and then get a tube amp for your mids and highs and get a class D amp (or something cheaper than what you have now that isn't necessarily class D) for the low end. Class D is efficient and if you're only looking at putting something on the woofers, that should work nicely. It could also be cheaper than just buying a tube amp now. Hope I'm making sense