Yes, 60db gain for the RR2150 is up near the top end of phono preamps I've seen and I agree you've probably maxed out the volume achievable with your current setup. I've always felt the greatest gains in sound quality occur by upgrading the ends of the chain, that is the speakers and for vinyl playback, the cartridge. Speakers are a big investment so the easiest item to replace is the cartridge. I think you'd find the Denon DL-110 or Grado Gold nice upgrades from your Sumiko. The Denon will have a more detailed presentation while the Grado will have fuller bass and both are just a bit more than you suggested you'd spend on an outboard phono preamp. It might be something to consider.

I run a separate phono preamp through my RR2150 (Pro-Ject USB V), but only did so in order to be able to rip vinyl to my laptop. I think that the phono stage in this receiver sounds quite good and plan to switch back to it once done ripping my vinyl collection. As for low output, it has been my experience that the phono stage, regardless of system, always has a low,output than digital sources. I correct that with the volume knob!

Years ago when I broke out my turntable after it being in storage for 15 years I noticed that it didn't have the crisp accuracy I wanted and was pretty weak as far as SPL. I spent some time cleaning the stylus and realigning it's headshell and cartridge. It really fixed the problems and made the sound much better. If the problem still exists, give it a try. Besides anyone with a turntable loves to tweak it anyway.