I said "all you need to know". If you can't be very pleased with what's there, you will just confuse yourself further by scrounging around in the underbrush.
I suppose if you cared to limit yourself to their selection and their opinions then you could limit your resources to just one. You'd be missing out on some of the finest options in amps and headphones though. Some of the most respected names in headphone options are entirely missing from the Headroom roster. Just from the top of my head the 'AWOL list' includes such luminaries as Eddie Current, Leben, Luxman, Woo Audio, RSA, Meier, Yamamoto, EAR, Apex Peak, Audeze, STAX, virtually all pro IEM solutions like JH Audio, Ultimate Ears, Westone... I'm sure there are lots more I'm missing, as well as a very healthy and impressive DIY scene that has very serious contenders in amplification for headphones (Cavalli,Stacker, AMB b22 m-3, and others). Don't get me wrong, I think Headroom is a good general resource, but you'd be doing yourself a disservice to think the world of headphone listening is contained within those narrow boundaries of what they sell and support. The alternatives are no more "confusing" than what is offered there, and I've got no clue why you'd suggest that offerings beyond that one dealer represents "the underbrush". That's a very myopic view of headphone listening. It's like suggesting that someone interested in high-end audio limit themselves to the resources of a single dealer who happens to stock a broad selection of good examples and suggesting that that is all you need to know about the high end - it's all right there at, say, Underwood Hifi (actually Walter has a great cross section of good gear - but so do a lot of dealers), and one need not confuse oneself with the rest of the options out there. Granted, the headphone world is even smaller than the world of high-end and options are more limited, but its a whole lot bigger than Headroom's offerings.