Setting up Quad 988 right

Having owned 88's I would move them farther apart if enough clearance from the side walls is available. I never tipped mine up or down but I did replace the factory feet with brass cones and discs. As you are discovering speaker placement is critical for optimum performance and it may take a great deal of experimentation to find the right spot.
What is your amplification? This is also critical.

Raising Quad 88 or 89 off the floor onto stands will severely affect (reduce) the bottom end. This is well documented not just my opinion. Search out the posts here, on Audio Asylum, or discuss with your retailer. Keep them coupled to the floor; as I mentioned previously the brass cones worked great for me. I had mine almost 4 years and tried almost every permutation of distance from front wall, side wall, toed in/toed out, tilted up/tilted down, etc.
Good luck and email out of this thread if you want to discuss in more detail.

Haipo,
That's interesting because I used 3 amps on my 988 and the Atma-Sphere was by a wide margin the worst regarding bottom end control. The impedance on the speakers drops to 2 ohms around 10KhZ if I'm not mistaken and OTL amps including AS do not handle loads like that very well. AS amps really strut their stuff with impedances avove 8 ohms. My M-60, however, sounded terrific in the mids. I also used a BAT VK60 for a while but the 988's in my listening room sounded best with a Pass X150.5 believe it or not.
Good luck!!