I feel like I want something more than the m40x. They are clear, but they sound little and not really powerful.
What exactly do you mean here?
Because "powerful sound" in terms of headphones or speakers can mean how the dynamic and loud the sound is, which depending on the perception of the listener, the deficiency for which can be due to a mismatch of sensitivity (and impedance) to power output (at that load impedance, and also distortion due to a mismatch to the output impedance) or outright response ie having the upper bass boosted to emphasize the beat which can both be a result of and also be 'remedied' by an impedance mismatch, or in the case of Grados, they really just have more of that boost but it's also coupled with very high sensitivity at low impedance.
Also by "little" do you mean the soundstage is a narrow and lacks depth? Just be aware that
1. Sometimes what some consider as "wide" is just overly emphasized treble that images the cymbals and ang guitars far out to the flanks disproportionate even to each other
2. Depth is very difficult to achieve in headphones. A combination of really good imaging and angled pads or driver mounts helps the AKG K70x and Sennheiser HD800 here, but can get a bit deeper with Crossfeed. Crossfeed though doesn't guarantee the same percentage improvement in anything else in terms of absolute depth, but in some cases can make everything proportional in distance. Either way, Crossfeed makes the whole soundstage seem 'narrower' as well but what it's doing is pushing the cymbals towards the center where they would actually be on an actual stage ie the drummer doesn't have 3m long arms.
Open/closed cans
First of all, I have no idea about which kind of headphone I should look at. I already have some closed cans, so would explore open cans, but someone told me they lack bass.
They don't really "lack" bass in the sense that if you measured them in a lab environment open headphones don't have that much less bass across the board. It's more of if you measured all the closed backs and got a result that suggests such the thing is it's not so much that the open backs lack bass but more of how the closed back headphones include headphones specifically designed to produce far more bass than is natural, like mass market headphones (Beats for example). And many closed back monitoring headphones would have measurements that are more in line with open back reference headphones - the difference here is that the closed backs get used by the artist singing into a mic if they haven't started using in-ear monitors yet while the open back reference headphones are what the engineer and artist will likely check the mix with along with speakers.
And even if you go by ear instead of lab measurements and find they really do lack bass, remember that again
1. The closed back headphone samples may be mostly geared towards producing more bass to begin with
2. If anything the closed backs will trap soundwaves coming off the back of the diaphragm, which, if it doesn't cause cancellation or damping issues, is due to using a driver that has T/S parameters designed to work in a closed enclosure (this is why you also can't just cover an open headphone or drill out a closed headphone), while also blocking a little bit more ambient noise whereas
3. ...the open back headphones will not even try to block out ambient noise. Basically, in a very quiet room, as long as the open back headphone isn't something that has a response that really nosedives before it hits 20hz and you also manage your expectations - headphones can't kick you in the chest the way speakers like the Sonus Faber Stradivari Homage will because you can't just override physics, ie two towers with two 250mm drivers each at 3m away will kick your chest in a way that a 40mm driver right by your ears never will - the open back headphone would still have enough usable bass.
I listen to metal, djent, progressive, and also heavy electronic. I dont listen only this kind of music, I don’t like overemphasis on bass, but a satisfying bass and guitar tone is required.
I've been using a Sennheiser HD600 to listen to Nightwish, Kamelot, Dream Theater, etc for over a decade now.
Driveability
This applies both to closed or open cans, would I need a dedicated device to listen to new pair of camps? I don’t want It to substitute my AK in anyway, it should only enable me to completely enjoy the new headphones and possibly add value.
Again, buying a desktop amp would mean the new headphone are dedicated to home listening, and that is fine.
Check your AK if it has a line out mode so you're sure it'll feed a fixed ~2V line signal to the amplifier. If it does then you can use that as a music server.
Due to Android USB Audio issues this year I've moved on to using a Hidizs AP80. It's set to Line Out so it fixes the output to what a line output should be (~2V) and then to handle ergos I just set it beside the amplifier and then control the AP80's playback using my smartphone (volume control though is still on the amplifier).
Questions for you
1. Should I look for a pair of closed back or open back? Which kind of headphone and why?
That depends on you really.
I don't listen with my open backs earlier than 9pm that way there's not much noise coming through the window. Unless I run the A/C, where the noise is at least a constant hum after I get the room cooled down in the first 15mins of it running full blast with an industrial fan sucking in the A/C's output and throwing the cold air all over the room.
2. Given that I am willing to spend in order to get as much as quality as possible, without stepping in the zone where more money means meaningless added marginal value, which headphone should I look for?
3. If the can you are suggesting me needs an amp, which one and why you suggest?
Well I was listening to Kamelot using my Sennheiser HD600, Meier Cantate.2, and Hidizs AP80 last night.
You could look into an HD58X and Meier Jazz FF, maybe a DAC with SPDIF inputs if your AK has digital outputs and you're fine with not using the AK's built in DAC (assuming it has no line out; otherwise, just us the AK).
If the cans you are suggesting are easy to drive and can be driven by a phone, would suggest an amp anyway, and if yes, which one and why?
Maybe try a Grado RS2x and even a portable amp will work with that.
Problem is if you don't like it and you want to try something else the portable amp might not be enough for that one.