You will find that with all the accessories and/or attachments made for the power boxes, that any 'internal' device will certainly be a lot more pleasurable as compared to those external devices.
Internal devices allows for nice pleasurable contractions of the muscles, providing the setting isn't too high.
External devices will give a 'pins and needles' effect, much more uncomfortable, unless you're into that sort of thing. But for sure, can be painful.
Combine one internal, and one external device, and you will find that the sensations from the external device, will over power the feelings from the internal device.
And for sure, always start at a low setting. These little power boxes, even running off a 9 volt battery are powerful!
With an external device, you 'will' experience a lot of pain. But with an internal device, remember that current is contracting muscles. Besides the pain involved, I have no doubt that some of these power boxes and certain attachments, cranked up 'too high' could possibly tear a muscle, maybe even break a bone. The contractions can be that powerful!
And common sense, keep it below the waist, unless you have those special attachments that won't pass the current 'across' your chest. These power boxes are all based off a TENS unit, which is made to penetrate deep into muscle tissue. Which means that it could interrupt your heart rhythm, or stop it all together.