The general answer to your question is: You can do it, but it's a pretty bad idea.
Because the signal level required to power headphones is in the same range as a line level audio signal, and headphone amplifiers have volume controls, you can always turn the output of one amp down to a level where you can then use it as the input signal to a second amplifier. The problem with doing that is that the distortion and noise from both amplifiers will be added together. (You aren't in any way "adding the power of the two amplifiers together"; you're simply running the signal through both of them; but only the amplifier actually connected to the headphones is driving them - the other one is acting as a preamp.) How much of a problem this will be will depend on the performance of the individual amplifiers.
The other problem is that bass requires a lot of energy, so applying a huge amount of bass boost is going to increase the amount of power your amplifier needs to deliver, and the amount of power your headphones need to be able to handle without distortion. (And many amps, and many headphones, are going to sound really bad if you drive them that hard. You're basically in the same situation as someone who wants to apply a huge amount of EQ some other way - your equipment has to be able to handle it.)
The other thing is that no amount of EQ is going to add bass that simply isn't there to begin with (if the recording simply doesn't go low enough).
There are, however, several interesting things you can play with if you're playing audio FILES rather than streaming content. If so, then you can use an audio editor program to "pre-process" your files before you play them. There are lots of audio editing programs and plugins that offer really flexible and powerful processing and EQ options. (You might try out Adobe Audition and the Izotope Ozone plugin. They aren't cheap, but you can get a thirty-day free trial to play with.) There is also a type of plugin called "a subharmonic synthesizer" - which actually ADDS BASS by recreating subharmonics.... so, for example, if there's a 52 Hz note present, the synthesizer adds a 26 Hz "subharmonic" bass note. These actually WILL allow you to add bass that wasn't there before, but that is "harmonically related to the music". There are a few of them, and they work to varying degrees - you'll find several if you Google the term. (I wouldn't recommend them for serious listening, but they are fun.)
In general, the headphone amplifier is what drives the headphones, so you can't put a standalone EQ
between the amp and headphones (if you do, the EQ then
becomes your amp - because it's what's actually attached to the headphones). If you want to use a standalone EQ, then you need to put it between your player and your headphone amp (so they must be separate), or you need to add a headphone amp after it (or get some sort of combination headphone amp with EQ).
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A few questions:
Is it feasible to hook up multiple amps along the same path for additive bass boost? Ie, If I got a micro idsd but wanted more bass boost, could I hook up another portable amp with bass boost between it and my headphones for an additive boost, or is that just crazy talk?
Are there standalone hardware equalizers that would work between idsd and headphones?
Could I hook up a digital device, eg fiio x series, between the idsd and headphones for software eq?
I just really need something with spdif to analog converter and strong bass boosting for my gaming, music, and movie needs.
Does the bass boost on the ican knock the socks off the boost on the idsd?
I'm having a real tough time trying to burn a ~$500 hole in my pocket to setup up the best sounding, bassiest ps4/xb1/pc/note 4 system to use with my SZ2000s. The option of adding another amp with bass boost or a hardware eq would make the idsd a no brained for me due to is versatility and future proofness. I just live in the middle of nowhere and can't really test these things out.
I'll sacrifice portability for now if I must so I have something to look forward to everyday coming home from work.
Please help! I don't know enough about what these devices are all capable of. I may be trying to have my cake and eat it too.
