The way Slowbro/Slowking's defensive stats work, in terms of mixed sets, they can achieve basically identical stats. IE, if you are using a specially defensive Slowbro, you can get the exact same stats with a physically defensive Slowking-- except King has the better movepool. Massively special defensive sets are better with Slowking too.
With AV, the more you invest in Special Defense, the more overall bulk you achieve. Obviously, a Slowking with max Special Defense has the most overall raw bulk, but it is severely deficient on the physical side, which totally ruins the point of having a Regenerator abusing bulky water-type. Meanwhile, AV or no, Slowbro can achieve the exact same stats as a max physically defensive Slowking if it wants to.
Moreover, Slowbro's stats allow it to achieve a physical bulk that Slowking simply can't. If you need physical bulk beyond the point Slowking is capable of, you need bro. Whether Slowking is viable is all a question of how hard the physical attackers in the meta hit. Whereas Bro's viability (having an inferior movepool) relies on the physical attackers hitting too hard for slowking.
With Max Physical bulk, slowking gets you the following stats:
HP: 394
DEF: 284
SpD: 257 (386 with AV)
Slowbro can get these stats too, but if Slowbro goes max physical, it ends up with:
HP: 394
DEF: 350
Sp.D: 197 (296)
With AV, the more you invest in Special Defense, the more overall bulk you achieve. Obviously, a Slowking with max Special Defense has the most overall raw bulk, but it is severely deficient on the physical side, which totally ruins the point of having a Regenerator abusing bulky water-type. Meanwhile, AV or no, Slowbro can achieve the exact same stats as a max physically defensive Slowking if it wants to.
Moreover, Slowbro's stats allow it to achieve a physical bulk that Slowking simply can't. If you need physical bulk beyond the point Slowking is capable of, you need bro. Whether Slowking is viable is all a question of how hard the physical attackers in the meta hit. Whereas Bro's viability (having an inferior movepool) relies on the physical attackers hitting too hard for slowking.
With Max Physical bulk, slowking gets you the following stats:
HP: 394
DEF: 284
SpD: 257 (386 with AV)
Slowbro can get these stats too, but if Slowbro goes max physical, it ends up with:
HP: 394
DEF: 350
Sp.D: 197 (296)