The top of the line Grado headphones are $1000. The Grado headphone amp is $350. The amp doesn't have a tube circuit, but it matches their headphones really, REALLY well.
Here's another option ... and its cheap. Go onto Ebay and look for a Sherwood 7100 or a 7100-A Reciever. These were made in the early 80s ... and the headphone amp is really great. The 7100-A sounds like good solid state and the 7100 sounds like tubes. You should be able to pick up a very good one for under $50. Add a pair of cheap Grado phones ... like their entry level "60s" or the "80s" and you're in business. Oh, and you need a cheap CD player too ... so while you're on Ebay, pick up one of the early Sharp CD players. The last one I bought cost $28. You'll be surprised by what you hear.
Vi