Since you're level 50, I'd just rotate the subs and use auto-grant for each account. Keep doing that until lvl 70-80, where the tank starts really benefiting the best Prestige gear & augs. Then I'd keep the tank sub'd, and rotate the others for more auto-grant. Around 105 you'll have maxed all auto-grants, then you can use the sub'd time to max each toons available AA. You can get "ftp"accounts up to 20K AA or so and still use all those when you're sub runs out.
At high levels, tank benefits the most from sub/gold. Prestige armor (tier2/3 from most xpacs) plus AC augs (almost all prestige from recent expacs though you can build a FTP set if you want.) Plus making sure you have all the defense AA maxed as soon as you can.
For support classes and DPS, it doesn't seem to be as important to have top tier gear. Melee DPS can get a decent boost from ultra-rare weapons, but it's quite as big a deal as it was in the EoK era. Casters/Healers can do fine in tier 1 gear, though there are exceptions if you want like the nice insta-heal click or nuke staff from RoS final quests. As mentioned above, puller classes are a good choice for a permanent second sub since they do get beat on and get some survival benefit from higher AC/HP tier gear.