Benchmark AHB2 - To 'mono' or not to 'mono'

though they are temporarily sold out...

@wwoodrum - only you can say whether you need more headroom with your room/speakers.
My room size is similar to yours and my speaker sensitivity around the same - and truthfully a single AHB2 drove my ATC’s to high levels without issue. Nonetheless adding a second amp improved the sound in terms of dynamic ease and soundstaging.
My motivation was not only the extra headroom but, as previously mentioned, the placement flexibility that mono blocks offered - allowing me to tidy up my setup using short speaker cables.

As you probably realise from the specifications, unlike conventional amplifiers, there is no downside to bridging the AHB2. The error correction circuitry ensures the Benchmark’s distortion remains vanishingly low as impedance drops/varies. This may well be another reason for the AHB2’s perceived neutrality vs other amplifiers - i.e. distortion in other amplifiers typically varies with impedance (whether bridged or not).
Of course the output impedance does double when bridged which means damping factor halves - in the AHB2’s case dropping from from 350 to 175 (at 20Hz/8ohms). This is still 10x higher than a typical transformer coupled tube amp and is unlikely be of much/any audible consequence. Certainly I heard none in my setup.

If you get a second amp you have the option to try both bridged mono and vertical bi-amping - so why not? For bi-amping you will need an XLR splitter of some sort if your preamp doesn’t have dual sets of XLR outputs.
Both options have the advantage of dedicating a single amp per channel and using short speaker cables. The bi-amp option further allows a seperate amp for woofer/tweeter - though most of the work is still being done by the amp on the woofer, which still has only 100W/8ohms.
You will gain a slight increase in headroom with the vertical bi-amp option while bridged mono will give a significant 6dB more headroom.
With more sensitive speakers, where extra headroom is redundant, the bi-amp option may well be preferable.
I use bridged mono and bi-wire to my ATC speakers (allowing removal of the link at the speakers).

I’m not going to gush over the improvements offered by the second amp as in overall terms they are not massive. Nonetheless, in my setup it does sound better.
Only you can decide if it’s value for money.
Some people spend more on speaker cables than the cost of an AHB2 amp so, if you’re into expensive cables, you could potentially save money moving to mono blocks and short cable lengths ;-)

I’m not going to gush over the improvements offered by the second amp as in overall terms they are not massive.

But isn't it the incremental improvements that sometimes make the biggest difference? Take speaker cables for instance - a person who doesn't do this for a hobby might not notice much of a difference between a $400 cable and one three times its price, or even 10x the price. But for you and I, that might make a huge difference in the way we experience the music. That concentration and depth of image that improves the overall presence of the music makes the difference worth spending, to us.

Anyway, I'm encouraged by your experience with two of the amps. (And have always wanted to listen to ATCs but have not yet done so.)

Yes exactly - personally I wouldn't go back to one amp.
I just didn't want to create unreasonable expectation about the addition. I do think the improvements extend beyond merely providing more headroom. 
As long as you understand this is an incremental improvement, rather that some sort of paradigm shift.