Thanks for that- I have a b800 waiting for me and am looking for it's cab partner when I return to the states in July.One thing to consider also is which head you are using with the S15D. I wasnt impressed with it when I used Focus SA / sabddi with it. However, my Carvin b800 is deep and flat by nature so when mated with the S15D it really brings out the lows that might be missing with another head.
I think it should be also said that those ultra-lows, while they sound nice in your bedroom, dont do well in a live band mix. Those ultra-lows can just turn into rumble and smudge. Personally, I prefer a tight low-end.
Being an Alembic guy- the Bag End connection has always been enticing- I'll have a run at one in July/August.