First of all, I think the problem is Quad L11 speaker is too small for classic. jazz, light music should be no problem. so, amp no need to change since it's Pre/main already.
Second, I think ching should improve power supply such as power cord, power filter to improve the sound imaging. you can change the fuse of the power cord to audio grade one before you change the power cord.
Finally, RCA cable are also one of the significant bottle-neck for you case, What RCA cable you are using?