Speaker/room interaction is the single most critical element in achieving superior sound quality. Get the Nolas interacting with the room properly and your problem will be solved.
I use the Viper Signatures with the deHavilland Aries G - 30 watt SET amps. For the longest time I felt the bass to be loose but after several moves and tweeks the speakers are now wonderful with a tight well defined bass. Just keep moving them and sometimes an inch one way or the other will make a huge difference. - Jody. You can see a picture of our room here - http://www.brooklynaudioinc.com