If you want to stay under 1k, you can forget about the SR-007 or SR-009, and only pick on of the Lambda models.
'Pro' (580V) or 'Normal' (230V) refers to the bias voltage of the membrane. The amp must supply this voltage, along with the (balanced) music signal for the earspeakers to work, hence three wires per channel: two for balanced audio, one for bias. This is just like on a condenser microphone, which is a reverse electrostatic speaker. Read up on how electrostats work if this doesn't make any sense to you. Either way, for the last twenty years or so, Stax has only used Pro bias, meaning that you only need an amp with Normal bias if you buy very old vintage Stax. In all other cases you need a Pro bias amp or transformer box. Pro bias uses a five-pin connector, with a combined bias pin for both channels, and Normal bias uses the older six-pin connector. Note that you can plug in a Pro bias earspeaker in a Normal bias socket, which works but isn't optimal, but not the other way around, as that could damage the earspeaker.
I cannot comment on the differences between amplifiers and transformer boxes, as I haven't ever used the latter. Amps are much more popular. Make of this what you want.
If the sound of a SR-202 is any indication of the SR-207, then I say follow Ali-Pacha's advice and get a SRS-2170 system via pricejapan, or locally if warranty is important to you. It's incredibly good for the money, and can possibly make you never want to use dynamic headphones again. Remember that when importing from Japan via pricejapan, you need to replace the adapter of the amp to suit the voltage in your country. This is easy, but pay attention to the polarity of the plug. For more information, search the forums, since this has been explained many times.
If you want to go second-hand, the older SRS-2050 system which includes the SR-202 is very good. I haven't heard the SR-207, but own the SR-202 (for many years now) and have used it with multiple Stax amps, including the (bundled) SRM-252 amp. The amp is plenty good for the easy to drive SR-202 or other Lambda's. Stax tube amps are also a very good match, from memory a bit better than my SRM-717 to be honest.
What does the SR-202 sound like? A very spacious, open and dynamic sound, quite neutral, but with a slight punchiness in the lower bass, and a slight emphasis in the midrange around 2 kHz. The highs are incredibly precise and detailed, yet smooth and not overblown but (for me) in perfect balance with the rest. I am generally very sensitive to harshness in the upper midrange or highs and do not find the SR-202 harsh or too bright, but just right. For example a HD800 is quickly fatiguing for me. Downsides are a bit of a diffuse soundstage (like all Lambda's: wide but diffuse) and the mentioned emphasis around 2 kHz. Strong points are their overall tonal balance and effortless, clear, highly detailed sound without distortion anywhere. I hope and read that the SR-207 is similar, but unfortunately I haven't heard it (yet).
The 407 and 507 have a good reputation as well, but supposedly have a more U-shaped tonal balance. I have a SR-507 incoming, so can hopefully find out very soon. Don't know about the SR-307. Some say it's equal to the higher models, others say it's not.
If you ask me, Stax should stop making it. Then they have the cheap 207, a medium 407 and a top 507.
Conclusion: the SR-202 and/or SR-207 make a perfect entry into the Stax world and have an outstanding price/performance ratio. For these, the cheap SRM-252 amp or older variants are very capable; Stax tube amps recommended as possible upgrade.
Let us know what you decide.
I needed a spare set for a future build.
