I used to run my SVT-CL with a 5.3 ohm 6x10. It was fine, and I spoke with the ampeg techs who said 5.3 shouldn't be too much of a variance from 4 ohms. However, if you're planning on building a cab anyways, why not just build an 8x10? Of all the cabs I've ever played with an ampeg tube amp, the original square-back 8x10s are by far the best sounding (IMO, YMMV, etc) and should be fairly easy to build since it's a square

. A lot of the sound of those cabs has to do with speakers as well, but I've heard the that new eminence B810 speaker is supposed to sound pretty close to the original CTS square mag speakers.