I second Bodfish's and Igglebert's posts, you will have NO problems driving A6s with your Mono Xs and I know they will give you the sound you want. My S6es are not as good "bass" wise as the A6s but the treble and midrange are fairly similar to the A6s. You will love the A6s.
What's the difference/improvement? The S6e's don't lack control, maybe a bit more extension perhaps...
The S6es have plenty of control and my MonoXs drive them with authority and do so beautifully. I am extremely happy with them. However, the A6s do sound a little "deeper", but I was offered a very good deal on the ex-demo S6es and I couldn't justify adding another £1K or so to go up to the A6s. Diminishing returns etc and my ears decided the difference was not worth it.
Cyrus DAC XP/PSX-R/Mono Xs, Chameleon Silver Plus, Kimber 8TC cable, Spendor S6e, A/Express, Shure SE420, Isotek conditioner, Clearer Audio mains cable /// DACMagic, Audiolab 8000S/SX/CD, Odyssey 4, Mission 753, Tacima, English Pointer, Jack Russell