Ascend would be a horrible mate with Klipsch.
I think you'll definitely want to stay within the klipsch family of speakers. I also recommend he stay within the reference line because they're a cut above Synergy. It would be hard to match the quartets perfectly with any current klipsch speaker, but the Ref line will be pretty darned good, and as good as any.
You can do a dealer search on their website to find a local reference dealer. Even if there isnt a retail store close by, there will be somebody in your area (home installer, etc, etc,) who will sell you the speakers.
I agree with Buckeye Nut. It would be hard to timbre match a Klipsch speaker, as well as, provide a similar sensitivitiy\efficiency for linear volume tracking among the speakers.
Here are the specs if you are interested in doing your own research. A post on the Klipsch forums would also probably be a better place to pose these questions. I am sure someone has asked this very same question over there and probably even someone who has found an ideal solution that would provide first hand feedback.
Klipsch Quartet Specifications
frequency response 38Hz-20kHz±3dB
power handling 100 watts maximum continuous (500 watts peak)
sensitivity 97.5dB @ 1watt/1meter
maximum acoustic output 117.5dB SPL
max acoustic output 117.5dB SPL
nominal impedance 8 ohms
crossover frequency HF 7,000 Hz
LF 650 Hz
tweeter K-76-K 1" (2.54cm) Polymer diaphragm compression driver
high frequency horn 90°x40° Exponential Horn
midrange K-61-K 1.5" (3.81cm) Phenolic diaphragm compression driver
mid frequency horn 90°x40° Tractrix® Horn
woofer K-10-K 10" (25.4cm) Fiber-composite cone active / KD-13 12" (30.48cm) Fiber-composite cone passive
enclosure material Medium density fiberboard construction (MDF)
enclosure type Bass reflex via passive radiator
dimensions 31.2" (79.25cm) x 15.9" (40.39cm) x 12.25" (31.12cm)
weight 52 lbs. (23.6kg)
finish Finished Black
built from 1989
built until 1996