I will agree with MCN that going with a good Broker/ISO is the smart move. When dealing with an in-house sales rep, that sales rep is trying to sell their product, and their product alone. With a good, experienced Broker you get an objective Expert's opinion. The Broker basically plays Matchmaker, pairing the Business Owner with the right Lender. They will know every Lender's Underwriting requirements, which industries they prefer and how to navigate all of these areas for you. There are also, unfortunately, the bad brokers who just send an application to every Lender out there and devalue the file (not to mention your Credit score). You should look them up online before working with them and make sure they do not charge any type of "Application Fee" or anything of that nature. ISOs/Brokers get paid by the Lender only.

Regarding Stacking, it is simply when you take a loan/cash advance on top of an existing one. It can be a 2nd position, 3rd, 4th, etc. Like others have said, you will get a wide range of opinions regarding stacking. I personally think 2nd positions are okay, depending on the situation. If it is manageable for you to pay two Lenders at once and you have a use for the funds that will improve your business than go for it.
Example: you have $8,000 remaining on a $25,000 advance. A Direct lender offers you $20,000 at a 1.40($24,000 total payback), but you have to pay off your balance. So you are ONLY netting $12,000 and paying back $24,000. Interest on top of Interest. The other choice is an ISO or Lender that "stacks" or offers 2nd positions offering you $15,000, at a slightly higher 1.42($21,300 total payback) but you net the full $15,000. You net $3,000 more and PAY $5700 LESS. Which would you rather do?

I do agree with most that anything beyond a second position (I have seen 4 or 5 Lenders on people's Bank Statements) is excessive and can do more harm than good for you in the long run.