The bass boost kicks in a hefty amount of low end. Some music will really benefit from the extra bass. Roxy Music's Avalon CD is a bit lean on bass and really benefits from it. Other music is best enjoyed pure, without bass boost. John Mayer's cover of Bold As Love doesn't need it at all. The Still Warm CD from John Scofield sounds much better with bass boost on, now Darryl Jones' bass lines really come to life. Don't switch it on on Erykah Badu's Baduizm Live. I could go on (and on, and on) but you'll get the idea.
Bass boost is a really useful feature to have, but don't have it switched on permanently, unless of course you're a bass head. Remember that the default bass boost is ~9 dB, I opted for a slightly lower bass boost of ~6 dB and I'm glad I did.
I built my cMoyBB with an amp gain of 6, enough for my PX-100's. The overall sound is very good and can be tweaked to taste with a whole range of different opamps. Currently I've got a Burr Brown OPA2134 in it. I also have an Analog Devices AD8066 opamp for it.
It will have to burn-in for a few hours, and I'll have to box it up too.
The cMoyBB is fun to build and a great little project if you want to get started in DIY audio (gewoon doen Boudewijn!). It's easy to build, the assembly guide is clear. Apart from the Alps pot and jack-inputs, parts are fairly generic and shouldn't be too difficult too source. Run some simulations before ordering/buying parts to get the frequency response you want. If it's all gibberish to you, you'd better stick to default values. John Seaber is very responsive to emailed questions should you have some.
It's highly rewarding when this cool little amp you built actually works and sounds great. Highly recommended!