In my experience, moving from 15" cabs to 10s certainly tightened things up in the lows. My first 4x10 cab was a Crate (sealed), then I went to an Ampeg 8x10 (sealed), used a Peavey 410TX for a little while (ported and sounded GREAT, less punchy than sealed), now I'm back to two Ampeg 410HEN cabs. None have ever sounded "loose" in the low end, no matter what bass I was playing, or head I was powering them with. Power ranged from various black face and silver face Fender Bassman heads, Ampeg SVT, V2, V4, GK 400RB, various rack preamp/power amp rigs, and no telling what else I've forgotten.
Therefore, IMO it's all in the 10's. Preferably sealed cabs, at that. And please stay away from that Ultra-Low button... Those things are evil.