Problem I have had for a year that is just depressing me

I had the perfect system for my apartment... onkyo txsr605 receiver paired with a KEF 2005's, me and everyone around me was impressed. I upgraded to the 3005 series and after that my system just became flat. The satellites continued to impress; but the subwoofer had a pecular problem; it would "turn off" at a high volume(think when joker blows up the hospital in dark night), just when things would start to get interesting (I didn't have this problem with the previous KEF sub)... I would turn the sub off and back on, and it would all go away.

I thought i had a defective sub, so I went out and bought a definitive 250 watt sub from bestbuy, and the SAME problem!, at the same volume settings!...

So I have toyed with numerous settings, contacted sooo many people, posted on soo many threads and haven't gotten to the bottom of it. Do I need a better receiver? Why could the onkyo handle a 250 watt sub from the 2005 but not 3005 series or a definitive one? why would that matter since they have built in amps anyway? Should I just buy back my old psw2150 sub and call it a day?

I have had this problem for so long that I have just about given up on my sound system... what's the point of upgrading it if noone can tell me what's wrong? in other words, PLEASE HELP!

13 x 15 ft in an averaged size apartment. I don't think there is a way to adjust the pre-out for the sub on the 605, and I have changed the cable in the past.