Well, I wouldn't use this amp with HD800s, Ultrasone Edition 8s, or DX1000s (headphones that cost more than $800 or so). But as long as you are not considering buying one of their portable amps, I would consider using any headphone up to the HD600 or HD650 with it.
That said, I would just read reviews and impressions (if they exist) to get a sense of how the amp is supposed to sound and whether that sound signature is what you are looking for. You also want to know if the amp has any operational problems that other owners experienced with it. Best of all, try to arrange to hear the amp. Maybe some e-tailer will offer a 30-day trial period.
Finally, if it doesn't seem like the amp for you, you might do better to buy a budget Meier amp instead. His amps are well-built, affordable, and easy to sell used. In contrast, I doubt you will be able to sell a Firestone amp very easily used if it is not your cup of tea.
