Here's a bit of information. The periods there were interest rate increase were 1994-1995, 10 year rates went from approx 5.8 to 8.0(38%) and 1998-2000, rates went from 4.4 to 6.8 (54.5%).
I believe those are the period to compare levereged vs unleverged muny bonds. I believe the funds should have an average of A rated bonds. for the purpose of this I think we can ignore maturity and perhaps use market price and NAV. Any further thoughts?