By investments, I mean totally passive - dividends from stocks, coupon payments from bonds, interest from CD's, rental income.

Seriously, some borrowers actually think by sending payment with their monthly coupon that the foreclosure will go away.  

To buy 10% off Lowes coupons.

I find that between sears, home depot and lowes, there are usually deals on appliances that can be had using coupons and websites that give you cash back, which help ease the cost of the new appliance.

Hey @Luke Carl,I am using a company called Boomerangdm.com currently who sends out unique local coupons to my imported contact database once per month. 

You can also stack these with coupons for more discounts and you can purchase gift cards at a discount and then really stack it up.

Any of you had experience buying zero-coupon DST as part of the replacement of debt ?

Yep, I'm financially independent but have to conform to the rules of 30 credit cards, run up my spending into the hundreds of thousands while jugglng all this crap and revolving payment dates and clipping coupons on the side to save $648.

Prices look higher, but once you factor in all the coupons they do and the Kohl's cash, the prices are great. 

modelling financing valuation for mortgage REIT bonds/coupon is very interesting as well.