Thanks Jeff for the feedback from SpamHaus.
Also, I do agree with the philosophy that a little collateral damage from legitimate sources is O.K. if the network originally sending the spam is a known, flagrant, and unrepentant spam source. (How else are they going to be motivated to clean up their act?)
Given Raymond and other's suggestions, I have set up a new test. I now filter at the MTA level using list.dsbl.org. My next level filters using SURBL (on the message's body content only, of course). My third level is to SURBL filter on the Client's server IP address. Although, I haven't yet seen any traction on this one yet... I know... not "by the book" :)
Finally, (...and this is the point of this message...) my fourth level is to filter on the ClientIP using each SpamHaus feed separately. This fourth level then saves all blocked mail to its own folder and I'm manually checking these daily. (This is not as time consuming as it sounds because these are only those e-mails which made it past both DSBL and SURBL filtering, but then got blocked by SpamHaus).
Since I started this only yesterday, I have 26 SBLs and 68 XBLs, with ZERO FPs.
Of course, I have other levels of filtering (linguistic/heuristic) after these I've described...
I'll report back in about a week (or two) regarding how many spams/FPs SpamHaus has caught by then using this setup I described.
Rob McEwen