4 ohms vs. 8 ohms

I am expecting a Jolida 502 BRC. It has outputs for 4 or 8 ohms speakers. I plan to use my Usher X719 withit, which are 8 ohms. I also plan to run a powered sub (ML), I dont know the ohms. How will it affect the amp? Do I use the 4 or 8 ohm tap? Thanks
check the archive, this has been asked a lot, but bottom line is use your ears to decide which you like best

how it affects the amp I will let others more qualified inform you

The input of a powered sub is the input of its internal amplifier, which will have a very high impedance, 10K in the case of at least some of the ML subs. That means the sub will draw negligible current from the main amp, and therefore have no effect on the main amp.

My instinct would be to connect the sub to the same taps as you will be using for the main speakers, presumably the 8 ohm taps.

Although connecting the sub to the 4 ohm taps while connecting the main speakers to the 8 ohm taps might work equally well. The only difference that occurs to me is that using the 4 ohm taps for the sub would result in a signal level being provided to the sub that is about 3db less than what the 8 ohm tap would provide. That would be compensated for with the sub's level adjustment.

-- Al
If the powered sub works like a REL by allowing a power amp output connection (rare I think), the impedence is something like 100,000 ohms so it presents almost no load to the amp. Otherwise it's a preamp output signal to the sub so the power amp doesn't care, and neither do I (heh). I run a REL from the same posts as the main speaker feeds so it all remains equal, and I use the 4 ohm posts...my speakers are 8 ohm rated but (as many do) they dip lower, and the Jolida's 4 ohm posts seem to sound a little better.
I intend to run the sub on the same tap as the mains. That's how I have it now, with good results. But my current amp doesnt have 4 and 8 ohms taps like the Jolida so I wasnt sure which to use. Plus Im new to tubes. Sounds like its safe to experiemnt with both 4 and 8.
Thanks all.