Evaluating preferred stocks in beaten-down sub-prime lenders

[MooreBonds]

I have a few preferred stocks of the sub-prime/other lenders (IMH, NFI).

Anyone have any thoughts on when they might be a value play, given the relatively generous yields they currently offer (10%-12%)? I know that the underlying common won't likely have any positive earnings for several quarters, and that their rich book values are subject to implosion when their underlying mortgages go into default, etc...but are there any ideas as to what metrics to use to determine when some of the preferred issues would be worth a possible value play with not as much risk as, say, a B- junk bond?