Sounds to me as if your room is big enough and the 988s will be far enough from the walls. Based on everything I've read about them and my experience with my 989s, you could move them closer to the side walls.
My room is 21' wide and 19' deep, with a ceiling rising from 6-1/2' high in front to c. 12' at its peak a couple feet short of the back wall. My 989s' panel centers are c. 6-1/2 feet from the front wall; their outside edges are about 4' from the irregular side walls. I sit with my head about 10' from the centerline of the speakers, and they're about 9-1/2' apart center-to-center...about 7' inside edge to inside edge They are toed toward the listener slightly but 'cross' far behind me.
I had to experiment more with getting them farther from the front wall than anything else. Every time I moved them farther off the front wall, the soundstage got deeper. I now get soundstaging and imaging better than I've ever heard anywhere (but I don't get around much to hear other fine-sounding systems*). Playing my absolute-fave Boult/Holst Planets, the tamborine and xylophone sound as if they're about 50' behind the speakers.
I bought an Eminent Technology LFT-12 centerchannel speaker to replace my much-loved, modified Aerial CC3B. I installed it yesterday and began comparing its sound to that of no centerchannel speaker at all...what some call phantom-center. I thought my system IN TWO-CHANNEL placed sounds in the middle relatively well, that is, instruments and voices that I thought should be in the left-to-right center of the image were well focused in the center, not too big, not too small, etc. It's SO good that using my 5.1-channel system with phantom center sounds BETTER (with a couple movies, anyway) than using any centerchannel speaker.
So the 989s sound GREAT, at least to me and my golden-eared goodbuddy.
Let me know if you have other questions.
* I get around so little, I've never heard any system sound better overall than mine is currently sounding.
Make sure that the wall behind the speakers does not directly reflect the sound. Your speakers will sound best when the back wave is radiating off a dispersive surface. Three feet is probably a minimum for good soundstaging. You might also want to either bring your speakers a bit closer together or move you listening position back a foot or so. Side wall reflections are not a big deal on Quads. Be patient, set up with quads can take a lot of time and expirimentation before you get it down. The back wave is a big issue! Get that right and they are wonderful. They also love tube amps. FWIW my Quad 63's were set up in a 13x19x9 room, 54" from the back wall and 12 inches from each side wall with the listening position 11 feet from the centerline. I used light drapes, plants and bookcases on my back wall - Wife wouldn't allow audiophile wall stuff.
You wont believe it,I use my 988's in a room 12'x12'and I get state of the art sound.I have them on sound anchor stands(custom made)12"high,backed up by a pair of Rega vulcan subwoofers(speakon connectors).I also use a Asc tube traps in the corners behind the speakers.Speakers are approx 31" from rear wall(drapes and sonex)I have been a Quad user since 1985.I lived in Huntingdon England for 13 years and I had the oppertunity to visit the factory.In my opinion the speaker should be raised off the floor.Do not toe them in too much.