Actually, the Sovereign doesn't seem to have any photon torpedoes except in Nemesis, in which some of the launchers are literally bolted onto the hull in these silly-looking turret things. In First Contact and Insurrection she seems to have only the one torpedo launcher which is equipped with quantum torpedoes; for some reason this torpedo launcher either shares a space with or doubles as the Captain's Yacht, which means it can't be much bigger or more sophisticated than the E-D launcher.
It's worth remembering that the Defiant--which seems to have four forward torpedo tubes--is often described as one of the most heavily armed ships Starfleet has ever built. Considering those same launchers can be used for both photon and quantum torpedoes, I'm willing to bet there's no real difference between them and Enterprise can fire both from the main launcher. That makes the extra launchers in Nemesis kind of pointless.