Run Balanced and RCA at the same time?

Try it. I've run two sets of speakers out of my pre-processor. One balanced to one amp and the other single ended to another amp. This is so I can use one pair of speakers for two channel and the other for surround. I turn off the amp for the speakers I'm not using. Haven't had any problems, though I just started using a second pre-amp for the two channel because I wanted to see what tubes would do.
--Neil

I run balanced and single-ended out of my BAT VK3i preamp all the time. The BAT provides one set of balanced outs, and one set of single-ended outs. As I use Infinity IRS Beta speaker, I drive the servo woofer EQ box with the single-ended outs, and go direct to my mid/hi amp with the balanced. Excellent! Your preamp may not supply both outputs at the same time, you may want to ask the manufacturer to be sure.

Hey Beemer...that's exactly what I want to do...run 15 ft balanced from LS 25 to ARC vt100 then run 15 ft single ended to my xover. The cables will probably be different.....

WireWorld-Jenna-Acoustic Zen to the power amp(balanced) and then my older 330 MIT rca to the crossover..sound ok??

Keep in mind that balanced outputs are TYPICALLY of higher output than most RCA based outputs. In other words, you may have to play around ( even more ) with gain matching the two amps. Sean
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I would keep in mind what Sean says. You will need to level-match as the balanced side will run aprox. 6db hotter. In my case the Infinity servo xover has a woofer level control. If your crossover has a level control you'll be all set, or you may have to make a "little black box" with a good pot!