Well typically from a general safety standpoint you're better off trying to find a way to get more out of the same drug(s) you've been using instead of simply moving to something stronger, but amitriptyline and really all tricyclics are kind of weird medications with lots of interactions and being on it kind of muddles the equation IMO.
The short answer to your first question is that the next reasonable step would be something like hydrocodone or even oral morphine, oral hydrocodone and oral morphine having a 1:1 equianalgesic equivalency (which isn't necessarily the same as an equi....recreational... equivalency). There are drugs whose potencies are more than what you've been taking but less than hydrocodone or morphine, but they're not going to be easy to find. All opioids have the potential to be addictive and dangerous of course, but I do think the step from codeine or even DHC to the "next level" is probably something to be avoided if at all possible, as it's a big jump in potency and again, common sense would usually say that if there's a way for you to safely continue what you've been doing it's probably better than doing stronger drugs. Imagine me making a balancing I-don't-know-what's-better-because-of-the-amitriptyline motion with my hands here, though.
A really simple way to potentiate most oral opioids is by taking an antacid about 30 minutes beforehand. It may not be quite as effective as enzyme inhibition for some people, but then again it can make a pretty big difference. It's certainly worth trying.
Promethazine is commonly combined with codeine to increase the effects and if you can find it, prometh or another first-generation antihistamine shouldn't be
too terrible to take with what you're already on.
Remember that White Grapefruit Juice doesn't work with codeine.
Plugging could be viable for the DHC, though any method that decreases time to onset and increased absorption etc. comes with its own inherent risks and dangers.
Kratom may be an option (as opposed to the DHC or codeine), but it's not one I can speak much on.
Of course there are many depressants that could be added *in theory* that could boost the opiate experience but I really wouldn't recommend it with the amitriptyline, which already sort of has an interaction with both DHC and codeine or any depressant. I always really thought that most tricyclics and certainly amitriptyline were CNS depressants but I'm having trouble finding any solid information that explicitly corroborates this other than "somnolence" being reported as a common side-effect, which really isn't the same thing. With potentiators in general you do want to make sure that you're not potentiating the amitriptyline too, but I don't think there's a lot of crossover (don't quote me on that).
I did want to try and give you whatever information I could specific to your situation as a result of the amitriptyline which does need to be taken into account, but otherwise I'm sure the basic questions of "what's next in strength/potency/recreational value from codeine or DHC" and "what are good potentiators of codeine or DHC" have been asked and answered here a whole bunch.
Here's one archived thread on DHC potentiators, which does indeed discuss some of the above and other methods.
Then there's also the
Opioid Potentiation Megathread.
Like I said, I think you need to be mindful of the amitriptyline but otherwise your answers to the potentiation question should be there, and the others should be pretty easily found from other sources besides me.
PS: I saw your other thread in BDD which is very, very similar to this one. There's no need to post the same basic question more than once. Also, while it didn't come up here, it did come up in the other one: sourcing of even legal substances or paraphernalia is not allowed on BL. This is stated explicitly in the BDD posting guidelines and elsewhere.
I'm barely any older of a member than you. I just came from another similar site and knew to read the rules, and what rules are typical for sites of this subject. 
