The LD1+ is designed to drive low z headphones, so yes it has enough power.
This is a reliable vendor for 6HM5:
And yes, there are much better opamps available. The best I have found to date is the MUSES02, and here is a reliable vendor:
The 6HM5 offer the warm sound i'm looking for? how is it against the 6ZH1P-EV? I've read it's also a good tube for this amp.
The MUSES02 really worth it's price? it's more than a third of the price of the amp itself.. with the cost of the upgraded tubes and opamp, maybe i can get a better amp?