Uncle Erick, how would you say the Smack compares to my X-CANV8P (that is if you have heard the X-CANV8P)? I've been eyeing them up for a while and was unsure if they are worth it. If it easily beats out my X-CANV8P then I'll have to give it a try. I'll have no trouble in building it at all so there's no issue there.
But OP if you go OTL with such low impedance headphones then you're going to get a mushy sound. The bass becomes too pronounced and also gets very flabby. Midrange is overly colored and highs roll considerably.
I would not use a 25 Ohm headphone on an OTL. All of them (save for the Zana Deux) have output impedances higher than the headphpne impedance. Look up "damping factor" for a better explanation.
You will want an amp with output transformers to get a nice, low output impedance.
I'd consider the Bottlehead Smack. About $500 and Bottlehead is terrific. Some assembly required, but get an iron and tellmyourself that you can do this. It comes with all the parts, instructions, and Bottlehead has a forum where you can get help. You can do it.
Bonus: you'll learn how to build and will be able to make anything you want on the cheap.