A pre/pro is simply just that, a pre-amp and processor. It doesn't power the speakers.
An external power amp is usually connected to a pre/pro so that all an amp will do is drive the speakers and nothing else. The pre/pro handles all the signal processing, decoding, video passthrough and so on.
A pre/pro + power amp setup is usually the best way to driver power into the speakers. As already explained by bro dirtrun, the power specs on receivers these days are mostly bs marketing. Receiver companies love to inflate numbers by rating the power handling at ridiculously high THD or at low impedance, and in Onkyo's case, rate it at 1 channel driven because bigger number = better right???!
Then they have all these stupid specs like PMPO or JEITA (yeah Yamaha, looking at you) which makes absolutely zero sense.
At the end of the day, once you get to a certain decent wattage, it is difficult to go higher to get more volume. Example, if a 100w per channel amp can drive a speaker up to 85db output and peaks of 110db, it'll take more than TWICE the power just get another +3db. Volume is not linear, it's logarithmic.
So don't be too invested into the whole power ratings game on receivers. A receiver with 100w into 2 channels at 8ohms, under 0.010% THD isn't going to sound louder at all in comparison to an 80w into 2 channels receiver. 20w is nothing.
The final thing to understand about the power tests done by sites like Sound and Vision. They do full-channel testing which shows certain receivers (particularly Yamaha) having much lower wattage than its competitors. But a full channel test is not a real-world representation of how power is used because there are ZERO movies, games or music that will blast speakers with the same signal for more than a minute. Power is always dynamic, volume is always dynamic.
The ultra center is as good as the bookshelfs, they are equally matched. Same sized drivers, same refined tweeters, a very decent 4" midrange just for the vocals. Best of all, it adheres to the best center channel design which is a 3-way WTMW, off-axis lobing will not be an issue vs the more common MTM design.
You do however will need a decent amount of space for the ultra center, it's dual ported at the back so you're going to want to have a good 2-3 ft. off the wall. It's also pretty hefty, so the use of sturdy stands are recommended.
That said, why not just get a pair of ultra bookshelfs first? Get the center later. I kidd you not, the ultra bookshelfs to my ears are equal and in some aspects better than more expensive well reknnown bookshelfs like the KEF LS50 or the Sonus Faber Venere 1.5. I've compared them for lengthy periods, I always go back to the ultras at the end, phenomenal speakers.
Thanks for the input. I'll go and demo the svs ultra later.. Seems like this svs is really an excellent speakers. But I don't have big space for HT, the best I can make is about 1.5 ft off the wall.