Speaker Upgrade

Have you heard the Response 2.5? I heard the 2 in 1996....nice speaker ,but not if you need solid LF performance. At that time the 4's were the full range speaker.Do a search and you will find a pair of 3's in your price range.I bet you don't want that pair due to age(10 yrs.)
Before you take the bath on selling the Quads,is there another subwoofer (Sunfire,Vandersteen,Hsu etc.)? Unless you feel the Quad is just not gonna cut it,trying to integrate a subwoofer will offer a decent upgrade path. As a last resort,do an Audiogon search for Audio Physic Virgo III.
It's a little over budget,(No affiliation with the dealer!!)but this is one of a handfull of "deal" speakers at something like your $2500 budget.

We have similar components. I had PS GCP 200/GCPS and sold it after listening to the XLR variable outs of my 168. I also had GCA 250 with Underwood Level 2 mod. I used it to drive my Magnepan 2.5R bass panels and another amp for the ribbons.

I am now using a Wyred 4 Sound MC 250/500.

I use an active XO.

I found speaker cables and interconnects made a BIG difference. I now use Clear Day solid core double shotgun silver and a mix of PS Audio Transcendent Silver XLR and DH Labs Revelation. Power Cords- Tek Line Cables new Reference series- both the micro and the micro Ref Extreme.

You might try the cable route first, as you will need them no matter what speakers you send up with. Maggies are unbelievable if you have the room. The reference 3A Veena is an outstanding monkey coffin. Personally I am addicted to ribbons/planars.

Vandersteen 3A sigs will work in your room and with your amps! They are definitely a step up, can be bi-amped and upgraded with 2Wq's if you get the upgrade bug again.
I ran; Ray 168>Pass X2.5>Pass X-150>3A Sigs and have "upgraded" everything out of the system but the 3A Sigs, they were upgraded with the addition of two 2Wq's. The point is the 3A Sigs are good enough to be a core component that you upgrade around.