While I haven't used these cabs, I have had a crack at the Beta 402 cab (using a Rickenbacker 4001 with the Rick-o-sound sending the treble pickup into a Coliseum Lead) and I find that it sounds a lot better with that cab than others. For example, I also tried that setup subbing in a generic 412 with JBLs as well as a Marshall 412 and while it didn't sound bad I definitely preferred the Sunn cab the best.
If you get a chance, I would suggest trying out a 410 for the highs, I was going to build a proper cab for some Transducers I had laying around but got impatient and just put them into a bass cab (using some sound foam to block out the port) and the results were actually pretty nice.
Though it may be psychosomatic, I think the Sunn speakers make a big difference.