Hafler XL600 Vs Hafler 9505 Much difference?

Well, my XL-600 is having problems in one channel now, so I will be taking it to a repair shop. Maybe I will buy an upgrade kit for it as well. Hopefully, the Amp sounds more like the 9505 then?? Thanks for your feedback.
I have never heard a Pro5000 before. I will check out prices on that.
Thanks

Chances are good that what you're hearing as a, "bass problem" with the 9505, is a case of expiring power supply filter caps. I've been using mine to power the bottom of my actively bi-amped system, for a number of years. My filter caps went south, about three years ago(symptoms appeared earlier, of course). The differences would be much more noticeable(across the board), if the amp was used full musical bandwidth. Especially given that you seem to enjoy cranking your tunes. These made a world of difference, but you have to leave the top off the amp, as they are slightly taller than stock: (http://www.digikey.com/product-detail/en/B41456B9229M/495-4228-ND/2269437)

Should have mentioned; those caps are slightly(10%) over the stock uF value, very low ESR and long life. You can further improve your 9505's presentation, with a pair of these(easy to install) FRED bridges: (http://datasheet.octopart.com/VBE17-06N07-IXYS-datasheet-143416.pdf), higher quality binding posts and RCA's, all of which can be gotten from Michael Percy: (http://www.percyaudio.com/Catalog.pdf)

Ok, assume I know nothing about audio, as that appears to be the case. :) What is a FRED bridge?

Also, isn't leaving off the top of the amp dangerous?