Originally Posted by DiabolusInMusic
In order for you to run two cabinets in series you will need a special cable or a junction box, that will create a 16 ohm load. Hooking up from amp>cab 1>cab 2 or one cabinet into both outputs from the amp will still be a parallel connection.
If you insist on running the 2x10 with a 1x15 ensure you put the 1x15 on top so you can hear it start to struggle before the 2x10. You will not find any positives to mixing cabinets only negatives. Plenty of guys will say "it works for me" which it might just fine, but there is plenty of evidence against mixing cabinets.
Most 210 and 115's are matched pretty well with regard to both power handling and basic mechanical capability. You get into a bit of trouble with a 410/115 though, per your comment above.
Phasing and other issues are relatively trivial with most bass guitar backline situations IMO and IME.
That being said, no downside at all to stacking matching cabs, and some possible, if relatively minor downsides to a 210/115 stack.