The trouble with most OTL amps is that they have an output impedance above 32 Ohms. You need an output impedance below the headphone impedance. Otherwise, the power transfer gets bad and you'll have poor bass control.
Traditionally, you lower output impedance by using output transformers. Problem is, output transformers are expensive. Decent ones start around $100 each and go up from there. So if you have $200 into the output transformers and $100 for the power transformer, you're not going to be able to add the caps, resistors, potentiometer, jacks, wire, and case for another $50. You certainly would not be able to include labor and profit, either. This is why you don't find tube amps with output transformers for much under $1,000. If you wan to DIY an amp with output transformers, you could pull it off for about $500 in parts at the low end.
Even then, you might want to spend more. I've got a Ciuffoli SESS in parts. That was about $700 for the power transformer, three chokes and two output transformers. I have another $300 into case parts, jacks, screws, solder points, switch, power indicator light, knob, wire, and other parts. I need to spend another $200-$300 for a good stepped attenuator. Then it'll be 30-40 hours of labor to put it together.
The one commercial OTL with low output impedance is the Zana Deux. The 6C33C has a nice, low output impedance. That's what makes the Zana Deux special - you get the OTL sound at low impedance. I think Millett has a low impedance OTL design, too, but you have to build it yourself.
Impedance aside, the cheap tube amps cut a lot of other corners. The most important part of a tube amp is the power supply. It puts DC directly into the circuit, so you're also listening to the power supply. The cheap amps run the AC through some diodes and a cap/resistor network or two. That's usually not enough to get all the AC ripple off the line. So you end up getting a slight waver in the sound. Likewise, a lot of them run AC to the filaments in the tubes. So instead of steady DC power for the tubes, you have fuzzy AC. This is why the cheap amps don't sound very precise.
If you want a good power supply, you probably want to run a tube rectifier. This can require its own power transformer, which costs more. You'll probably also want to really smooth out the DC for the circuit, so you'll want to drop some chokes in there. Good power supplies have one choke coming out of the power supply which is then filtered by a cap/resistor, then into another choke for each channel. Then you'll want to run DC to all of the filaments, which adds even more cost. Some power supplies even add regulation to ensure that the DC never varies.
As you can imagine, adding all of this costs money. Parts, labor and sometimes a second chassis just for the power supply. It really adds up.
Construction also plays a big role. Cheap amps are on a PCB. Those are OK, but inserting/removing tubes can weaken the solder joints. It's easy to lift a pad if you have to replace a component - that requires major surgery to fix. Tube heat can also lift a trace, which also requires major surgery. You're better off with a point-to-point amp. But those require a lot more labor to build and terminal points/strips cost more than a PCB.
Solid state costs a lot less to do right. They operate at much lower voltages, so you don't have to buy the expensive iron for the power supply. Further, the low voltages let you use chips and other inexpensive parts for really nice filtration of the power. Chips are naturally low impedance, so you don't have to buy expensive output transformers. Also, PCBs work fine for solid state. It's easy to heatsink chips, too. So if you want a good design with an excellent power supply, you can do solid state for much less than the equivalent in tubes.
This is why I don't recommend cheap tube amps. There are too many corners cut in the design. A $300 Dynalo will get you cleaner power to the headphones. If you are willing to build or buy a quality tube amp, however, they are very rewarding.
