OK, then the SL900 is 900W at 4 ohms and the UL2 310 peaks at 750w at 4 ohms creating substantial headroom.
(and I sort of understand that headroom isn't necessarily more amp watts then the cab rating, but for general purposes the above combo has sufficient headroom.
yes?
I'm looking hard at purchasing a SL900 and want to know a good cab match.
Think of cab impedance relative to solid state amp output power like water coming through a hose. The more you turn the faucet the more water comes out. So, the lower the impedance of a cab, the more power is generated and applied to the cab, on a continuous and relatively linear scale.
Of course, cab power rating is only a loose guide to maximum power that you can apply to the cab without doing any 'heat damage' to the voice coil. Most heads rarely put our their maximum power (and if they do, it is typically in brief peaks, which is no big deal), and most cabs will 'fart out' (i.e., reach their mechanical driver limits) long before the voice coils are damaged.
So, the power matchup with the Streamliner900 and the 310UL is a good one, with the Streamliner 'running' at 5.3 ohms (so around 90% full power or so), and the Epifani's very high quality drivers being able to use all that power (i.e., the UL's have very good mechanical specs that can use almost all the power they are rated for thermally).
That was my typical long winded way or saying 'yes, good match-up regarding power'. Whether the 'smooth, fat, polite mid response of both the head and that cab are 'too much of a good thing for you' can only be answered by you.