I have successfully overwintered fairly large passi's (12" pot, 4 to 20' vine, coiled) inside by simply getting a 22watt 'circline' flourescent bulb (they look like this and hanging said bulb above the plant.

You can simply leave it plugged in, or put it on a timer- But I wouldn't go much below 12 on 12 off- 18 on 6 off being about what I'd call "average".

50's shouldn't cause it a problem.. It may grow verrrrrry sloooowly in that basement however.. Which might not be a bad thing, come to think of it.

I've got a number of passion flowers that I overwinter indoors. Among these is a Lavender Lady (which is confused for Amethyst, so I can't be sure it isn't).

I bring all mine in and put them in my son's room. They sit in front of a south facing window and I water about once a week to every other week. I lose leaves on some, some do fine and even bloom well indoors. Lavender Lady is one of the better ones for me inside. It generally will bloom as spring arrives indoors and it generally loses less leaves than some of the others.

~Chills

btw...also growing 'Incense', Vitifolia, Caraeula, incarnata, edulis and a couple of unknown ones indoors over the winter.