Hi charnish,
Many years ago, I started with a Crack and have since then built another Crack for a friend and I have built a few additional Bottlehead kits. I don't have any super-expensive headphones, and my favorites are the Beyer Dynamics DT880 (600 Ohm), DT770 (250 Ohm), and the Massdrop version of the Sennheiser HD650. I also have the ORA Graphene headphones. I don not consider myself an audiophile, and I refrain from exalting audio products in flowery language...
I just like the sounds of various Bottlehead products, and I enjoy building them, too. A few years ago, I was in the same situation as you described. When I looked into additional headphone amps and what they would bring to the table, I searched and read through posts in the forum and ultimately decided against the marginal improvements over the Crack with Speedball that I would get from the C2A, and went with the Mainline instead. The Mainline has a slightly different flavor and can handle any headphone, including those with a high impedance. I still stand by my decision and enjoy listening to both the Crack (with Speedball) and the Mainline. Just my 2 cents...
Cheers,
FS