Hi Greg,
Its funny you should ask, as we goofed on our inventorying last week and had to send off our last set of SPA 250's to a customer until we get more in the mail this week.
I did have a few people (old and new to our speakers) come by to listen in the mean time with the SA100's we had lying around and played them with single and double subs and I did find myself mentally comparing notes on the two sub amps.
There are a few differences in the sound, however the gain and frequency settings are almost identical between the two amps.
- 9 o clock for the gain
- 10:20-10:30 for the frequency
This is the case whether you are using single or double subwoofer setups. These settings are definitely negotiable - I've got a setup in our guest bedroom at home which is a heavily carpeted room and that sub amp needs a solid bump on the gain knob to sound right in the mix.
Now, while the two amps have almost the same measurable bass levels on my RTA when they are given the same settings, subjectively I feel like the 250's have more headroom which results in them being harder to localize in the overall mix. It feels weird to admit that 150 more watts to the bass speakers results in ANY discernible difference considering that we are still using just 10 watts on the full rangers. But - considering that bass power consumption goes up exponentially as we drive down in the octaves, and we are mating conservatively rated class A+ tube power with typically optimistically rated class AB solid state... then there you go.
One other difference is that the SPA250's do not have a turn on "thump" like the SA100's. This was more for an annoyance factor than anything. For apartment dwellers already concerned about bass travelling I don't want to give any more unnecessary "door slams" every time they turn on their system. Though this is avoidable if you just manually switch the amps off with their power switches first.
One other "convenience" reason is that the SPA 250 is international voltage compatible, whereas the SA100 is not.
The crossover slope is a bit different as well. The SA100's have a 12db/oct slope, whereas the SPA250's have a 24db/oct slope. This seems to result in slightly fewer upper bass frequency cancellations between the Orcas and Subs, though this has proven very difficult to measure.
Now - the original reason that I opted for the SPA250's was when I started measuring the roll off characteristic of the Orca fullrangers themselves when used with a pairs of subs, and I found that, once broken in, the Orcas themselves were steeper than I had previously realized - hovering around ~24db/oct high pass.
So the 24db low pass and ~24db high pass creates a matching roll off character for the speakers - usually a good characteristic for a formalized 2-way system to have.
The thing is, though, that the single sub setups are a lot less sensitive to the SA100 vs. SPA250, actually. I'd officially call it a toss up between the two sub amp models if you are using a single sub. But for the double subs, there is a sonic advantage to the SPA250's because of the crossover slope.
Along with the Y-cables, which do away with one more mechanical connection between the amps and speakers, all of this has resulted in a really nice uptick in the overall quality of bass to our systems these days.
If folks are interested in getting the 250's and Y-cables, by all means do so. It is a pretty modest, yet solid upgrade. (And you don't have to ship your whole system back to do so).
Now - one "free" tweak I wanted to make everyone aware of is that the 400hrs+ on your drivers that Jim Rebman suggested over a year ago does do something tangible.
Yesterday I pulled one of the "shop pair" of Orcas that have been played hard every day for ~6 months overcoming tool noise and dishing out Jimi Hendricks, Dub Reggae, and the like. I hooked them up to our big rig upstairs, and holy cow do they sound nice! These are bona fide fatigue-free speakers for long listening sessions. The difference between the ~10hr to ~50hr short term broken in drivers we regularly ship to customers and 1000 hour long term broken in ones like these (same exact cabinets) is just shocking. Now - maybe the BeePree + Paramounts might be helping to illuminate this difference in a way I haven't heard yet... but that's a whole 'nother story for a different thread.
Cheers,
Clark
visual perception of value and all that BS. 
