All the new modern power boxes are based off the TENS unit. My first TENS unit, prescribed to me after a back injury originally cost $800.00, and had so many settings to it, that you would spend too much time just figuring out which was better.
TENS units began being made a whole lot more simple, and much cheaper as more and more of them were being prescribed by doctors. Eliminating a whole lot of those settings to just a few, more mass producing, and prices came down to around $80.00.
The first power boxes that hit the market, were of a simple design as well, having just a couple settings, and of course intensity and frequency adjustments. Folsem was the first big name in these, and they did sell a ton of them! They were much more expensive though, now in the $250.00 to $350.00 range.
Other manufactures followed, PES came out with their own power box, trying to make it exclusive only to their accessories. At first only manufacturing accessories for women, eventually came out with a line of products for men.
Naturally third party companies hit the market with accessories that would be compatible with most of the power boxes on the market.
For a very long time PES only sold direct, and not through any distributors. That had changed just a few years ago and naturally their sales have increased quite a bit.
With some companies specializing in products for women (like PES) and some for men (like Folsom), and third party companies making products for either, there became a market for 'adapters'.
And now there are adapters on the market to go from one manufacturer's products to another. Yes, even an adapters to connect a TENS unit up to those accessories, or TENS unit sticky pads to the power boxes. Some adapters can even be found at Radio Shack, such as those going to and from the mini banana plugs to micro banana plugs. But my personal experience with those were that they were very intermittent! I wouldn't recommend them.
The latest and most sought after power box for the last several years is called the Eros Tek. They do have several models, smaller more portable, similar to the Folsom and PES, one with a remote control and their premium, a fully programmable table top model. With more features, then I care to list here. But comes with a CD for PC programming of the unit.
Being in the 'industry', I hear a lot of feedback of such products, (as well as owning more than I would care to admit to!). I had heard the the PES power box isn't as good as a Folsom power box, or the Eros Tek boxes, even the cheaper ones.
Thus dealers don't push those as much as the Folsom or Eros Tek. With adapters readily available, one is not limited to only purchasing the same manufactures accessories, to go with their power boxes. One can pretty much mix and match whatever is desire these days.
So if you have, say a Folsom power box, you can connect PES accessories to it, and visa versa. Same thing with a TENS unit, you can find an adapter to connect any manufacturer's accessory to it.
If you have a TENS unit that has 'lots' of those adjustments and settings on it, about the only power box on the market that would supersede that would be the premium Eros Tek 312b.