Thanks for the input, Vic. I'm still a bit unsure-Nick Epifani seemed to think when I spoke with him today that the 4 ohm 210 cab would be a better match for the 310 than the 8 ohm, mostly because he thought the 8 ohm cabinet would be too unbalanced volume wise. My amps both go down to 2 ohms, so I'm okay with a 2.38 ohm load, and since I would probably only use the 210/310 cofiguration less than 5% of the time, it might not be a bad approach. Any thoughts?
Assuming you are driving both cabinets WITH THE SAME AMP I think Vic was right in the first place about the 8 ohm version being the ideal match because of the 16 ohm drivers each drawing the same power, no matter what cabinet they reside in. With each of the five identical 16 ohm drivers drawing the same power, they would all reach their maximum power handling and Xmax at the same point. With a 4 ohm version UL210, its 8 ohm drivers WILL draw more power INDIVIDUALLY than the 16 ohm drivers in the UL310, but because of this
they will also reach their power limit earlier! This also means that you would be restricted by the power limit of the UL210
and would not be able to run the UL310 at its full power capability. This partially defeats the benefit of why you want to use two cabinets in the first place. While Nick IS correct in saying that there would be a better volume balance between the two cabs with a 4 ohm UL210, I don't believe this is necessarily a requirement, IMO. What you are after is overall
system volume. As you would most probably stack the UL210 on top, the 8 ohm version would be plenty loud to your ear in combination with the UL310, resulting in a composite 5x10". And as I said the upper power limits of both cabs would be reached at the same point where they could BOTH be used to their fullest (and safest) potential.
However, all this being said, if you drive each cabinet with a DIFFERENT head, it may actually make more sense to use the UL210 in the 4 ohm version. One reason is because amplifiers always put out more power at 4 ohms than at 8 ohms, and it's usually easier or cheaper to find an amp that puts out the power you need into 4 ohms instead of into 8 ohms. Another advantage is that neither amp would need to drive lower than a 4 ohm load each, where a single amp would!
One last point on this, and this is completely subjective on my part and IMHO. On a direct A/B comparison between my UL310 and a friend's UL210 4 ohm, each of the same series, I definitely prefered the tone of the UL310 in the mids. I'm not sure whether anyone here else has suggested this yet unless I've missed it, but I really believe that
some of the magic in the UL310's and UL410's of BOTH series is due to the 16 ohm drivers they use. When I originally auditioned my UL310 at Epifani's shop, I asked Nick about this and though he said the difference is slight, he admitted that it's definitely there. Even if I'm totally wrong about this, using the UL210 8 ohm version should be tonally more compatible because of identical 16 ohm drivers as are in the UL310.