OTL amplifier and electrostatic loudspeakers

Hopefully, Ralph will chime in, but in the meantime...
It looks like Quad says 100wpc/sensitivity at 86db, so your Atma's are a little on the low side of power. You should be able to drive them, but I wouldn't crank it up too high.
That said:
I would suggest buying a pair of Paul Spelt Zero Autoformers to handle any low impedance dips of the Quads-I see Stereophile measure a dip to 4 ohms at lower frequencies-not terrible, but why tax your tubes?
If you could relieve the amps from the lower frequencies with a sub/s, you'd probably be good to go.
Bob

Ralph @atmasphere on this board should be ultimate authority @gdnrbob so rightly said

I would try it with and without a set of ZEROs (www.zeroimpedqance.com).
With that speaker our customers seem to like both with and without. Overall the speaker does not have any severe impedance dips, so if you use a set of ZEROs don't use more than 2x.
The ZEROs would mostly be used to restore the highs - the amps otherwise should be fine in the bass. Make sure the speaker is at least five feet from the wall behind them.

@OP. 
Well, there you go...
B