as a fellow A- ranker, I'd say that you always want to have Ink Resistance and Cold-Blooded.
Cold-Blooded's usefulness is just insane: if you're echolocated, you're only visible for about 3 seconds. this is very useful not only in evading enemies, but it can potentially throw them off guard if they assume you're still visible on radar. it's also worth mentioning that this extends to point locators, essentially putting the Forge SSP out of a sub. however, this does not extend to disruptors: you'll still be visible if you're pinned by one of them.
Ink Resistance is also very helpful: when standing in enemy ink, you'll get damaged far, far slower. I just cannot tell you how many fights I've won with this ability: without it, you're dead meat the second you end up in enemy territory.
as for tips, I don't have that many. however, you should always check your gamepad. for starters, it allows you to track enemy ink, as well as the enemy making said ink if they try and over-extend.
secondly, it gives a massive boost to the Echolocator: while on the TV, all you can see are general directions pointing to an enemy, the gamepad flat-out shows you their location.
finally, don't forget about secondary abilities, especially if you have multiple pieces of gear with the same main ability. for instance: I've got three items with Cold-Blooded, yet of those three, the one I use the most is the anchor sweat, due to its high chance of getting all three slots with Damage Up. on the same routes, don't throw a piece of gear away if it only has one or two of the same secondary abilities: one of my main setups consists of my shiny Red Hi-Tops, (3 Swim Speed Up subs), my Retro Gamer Jacket, (2 SSU subs), and my Camo Mesh (SSU main, with one sub). 3 subs are equal to one main ability, and this setup gives me the equivalent of 3 main SSU's without having to ditch ink resist or Quick Respawn.
all in all, it's worthwhile going through your inventory to see what works, and don't be afraid to switch it up if you feel the need.