What to wear under tights with fabric padding?
TL;DR If it's got a pad, don't wear anything under it. - Benzo

I've owned several pair of cold weather cycling tights, including some that were questionably "cycling" tights because they had no padding/chamois.  I've always found it best to wear nothing underneath when there is any sort of padding.  If there is no padding whatsoever (just a tight) putting a set of bike shorts/bibs underneath is your best bet. For anything with a pad/chamois/additional material, any additional warmth requirements should be layered on top (long underwear, shell pants, rain pants, wind fleece, etc).

In your case, I may consider either getting rid of them in favor of something with a real winter chamois (should be thicker) or possibly try to remove the extra "pad" and use bike shorts/bibs underneath.  

Seat fit is a dicey enough proposition without mixing pads and hoping the combination is not worse than nothing.   

FWIW Disclaimer:  I wear bib shorts down to about 10F (usually with over baggy shorts or a wind pant up to 40F).  I wouldn't ever wear my winter bib tights at a temperature above 10F, so it's rare that I would ever want to take them off and have something underneath.  I won't ever buy a winter tight unless it has a thicker pad in it.  I'll take all the insulation I can get sitting on a -20F seat.  For that reason I find my lightly "race padded" bib shorts to be inappropriate for actual winter riding.