Just try a cheater plug first to see if lifting the ground is the issue before buying a DI box or other solution (or better yet solve your ground issue).
I've seen some discussions for use with subs where there's a concern with sub 20Hz filtering built into the XLS, and when I corresponded with Crown they confirmed as much (they advised "It’s a 12dB per/oct roll off. It’s not fully flat to 20Hz, roll off will start a little bit above 20Hz."), altho some who have done their own testing say they aren't seeing as significant a roll off to confirm. Wouldn't matter much in your use for your mains in any case. Not familiar with claims as to hum eliminators changing frequency response, never used one nor looked into them. You could check out the Crown XLi amps, they don't have dsp that you don't need but still retain rca connectors.
Many pro amps only have the balanced connections but you can easily use a converter cable. XLS is one of the low cost offerings with consumer friendly features such as the rca connectors and adjustable sensitivity, haven't looked at XTi for a while but seems they're rated similarly spec-wise (and think same Drivecore amp guts) but with more capable/useful dsp but without the rca/adjustable sensitivity. You can spend more for sure on an amp than on the XLS, but not a lot less...Behringer iNukes come to mind but then fan noise can be an issue (but if you have an equipment closet...). QSC could be an alternative as well and think they have units with rca inputs too...ask
@Verdinut about QSC, he uses some and is more familiar with them than I am.
Of course it's easy to find more expensive amps....personally I'd go for SpeakerPower, Powersoft or LabGruppen over my XLS for sub duties ideally but $$ and I really don't need the additional power either. For consumer brands Emotiva, Monoprice Monolith/Outlaw/ATI stuff might be a consideration. Parasound I think is overpriced but have some good stuff. I haven't looked around a lot lately either.
If you're using volume levels (calibrated I assume) on the absolute scale of 72-80 you like it pretty dang loud, so you just may benefit from a more powerful amp to at least provide a bit more headroom (keep in mind it takes a doubling of power to gain merely 3dB, and your speakers have sq limits, too). What issues are you having particularly at these volume levels now? What is the extra push you imagine?