I don't have an Ultra yet, so I can't comment on specific speakers that will sound good with the unit. I have supplied the PA for my band for about 10 years now, so I might be able to give some general guidance.
The PV215A is a very low-end speaker, so it is not that surprising that it doesn't have the sound you want. In general I would recommend a powered speaker rather than a separate speaker and power amp. The main reason is that good powered speakers have separate amps for the lows and highs (biamped), which can get rather costly if you build it with passive speakers and amps. The improvement in sound when going to biamped speakers can be dramatic, and you can be sure that the amps are well-matched to the drivers.
The workhorse speaker that I use with my PA, and plan to try with the Axe-FX, is the
Yorkville NX550P. I compensate for variations in frequency response using a dbx Driverack 260 speaker processor, and my 550P's have a nice flat frequency response requiring only modest compensation. Surprisingly, a cheaper speaker that I use for monitors, the
RCF ART 310-A, has a very flat response requiring almost no compensation.
I'm hoping one or the other will do the job with the Axe.
You can get some knowledgeable recommendations on the
ProSoundWeb Forum, where the sound guys tend to hang out. The link is to the LAB Lounge, which welcomes questions from newbies.