Wohoo! Let me weigh in on this. I guess I wasn't clear enough when talking about the MECI line filters in the first place.
1. The MECI line filters I am using are blocks. They are low pass filters. They pass 60Hz really good. They pass 100kHz really bad. I have never connected an audio generator and o'scope to one, so can not elaborate on the roll-off, but would guess 1KHz and above. No component values are given on the filter.
2.They seem to work equally well whether grounded or not, which is a hint as to how much signal sucking (or shunting the signal to ground) could be taking place.
3 " IN" and "OUT refers my connection of a two or three wire extension cords for input (plug end) and output (socket end). I don't have one that I can look at, but do not remember any contact lugs marked as IN or OUT. It is just a simple PI filter, so I don't think it gives a rip about orientation.
4. Since the filters do not PASS the X10 signal, they may lessen or stop the "signal sucking" effects some UPS seem to do, if the UPS is plugged into the filter. (Through the filter, those UPS's would actually look like a very high impedance to ground to 100KHz , rather than a very low impedance to ground). As you can guess I have no UPS.
I do have nine or so of these home brew line filters, mostly on devices with very noisy switching power supplies (Toshiba big screen, CFL torchires, microwave(s), etc. and in a previous home, a Lennox Pulse furnace).
BTW a synapse just fired and I remember something (any applause in recognition of this unusual feat is appreciated). When we first got the furnace, I could not get any X10 down that branch. After installing the filter in the furnace, the branch became usable to X10. I have two of the ELK signal level meters and there was not huge amounts of noise riding on the furnace branch, which caused fits of frustration in the trouble shooting. This decade old experience may have been my first experience with a "signal sucker". If it was, there is hope for you guys with sucking UPS units.
Going back to sleep now.