In terms of Monsters, it sounds to me that the Gold version may be a little more up your alley.
I do see the MTPG (haven't used the Copper myself) as a good all-around product but at the same time not a "wow, these are special" kind of product. These seem to be products suited for people who want a well balanced package that kind of does everything well and not much wrong.
I agree that BAs can be hit and miss in various ways, but not all BAs are the same. The right product will give you the sound signature you want. There are just a lot of "wrong" products for your particular tastes in the mix of what's out there that you have to weed through. Dynamics seem to be a little more "normal" for most people, expected signatures.
I'm not sure how satisfied you will be with the bass for a lot of products coming from the IE8. The IE8 does low frequency pretty well, and you just don't quite get the same quality of presentation on the low end with most other products. If you don't mind the midbass hump, the IE8 is a very nice product. I haven't used the Coppers, so I can't compare directly. I have used the Golds, and I do find the Gold's bass to be well presented and more controlled than I initially anticipated. For the number of people calling the MTPG bass heavy, I was surprised by how tame the low end actually was, and it didn't really overwhelm the mids that stood out plenty well. If you found the Coppers light, the Golds may be the better balance you're looking for. I'm not sure how the mids and treble are on the Coppers, but the Golds are pretty smooth. Maybe the Copper is edgier or emphasized somewhere. The Golds are pretty tame. Going back to the IE8, I too liked the mids and highs a lot, and the IE8 actually portrays the spectrum pretty well, only getting a little sluggish on the top end blurring some detail together.
If you want to piece together a few items to demo or buy, it sounds like you want to stick to the most balanced products.
The RE252 is my favorite, and you already own this one.
I also consider the Klipsch Custom 3 as a really well balanced earphone. Also it is a bit unusual in note for a BA, offering a thicker more textured note that is uncommon for a lot of BA drivers. Plus the sound stage on the Custom 3 is among the best, excellent size and linearity. If you want a BA but without the super clean BA kind of sound, this is one of the few that offer that. You've used the Custom 1 and 2. I haven't. I know Joker reviewed all 3 and really did not like the 1 or 2 much at all. The Custom 3 is a bit better though. While Joker ranked the Custom 2 at 6.0 in sound quality, he ranked the Custom 3 at 9.25 which should be an indication of the size of step up from what you're familiar with for Klipsch's Custom line. It's just a suggestion since you do prefer that thicker, more textured dynamic sound.
While I can agree with you in some ways about the highs with BAs, I'm not sure what you specifically like or don't like. A lot of the single driver BA earphones simply lack extension or end up pretty ragged up top (ex. PFE). Some BAs aren't very articulate, so you lack some of the thickness and texture, even up top, and you can get a sort of sparkly but not entirely meaningful high end. I feel the CK10 is a good example of what a BA can offer on the top end, and you're familiar with that. You've used the TF10, and I do very much like the TF10's highs too, although the sweet nature may or may not suit you. The emphasis of both the TF10 and CK10 really do require EQing though, but a Comply T-500 tip can soak up some of it in a pinch and tame these earphones down a little bit. The TF10 and CK10 are really the only two BAs I've used that I really, really like the high end, but I'm also a big EQ user and have a robust enough tool to do whatever I want and is needed to balance out a product. The only other earphone that rivals these two is the RE252, and the RE252 doesn't need EQing. These are the only 3 products I've used that offer endless treble and of a high quality. Most other products roll off, become ragged, or end up sluggish or or limited in some way to really accurately portray the high end. The IE8 is an example where the extension is there without rolling off way up at the top, but the bass geared driver simply can't keep up with the finer high frequency details. Stepping to something like the RE252 reverses the spectrum offering that top end and midrange but sacrificing some low frequency. I can't say I've used a dynamic that does the entire spectrum really, really well. I find multi-driver BAs to do better, but I find additional EQing needs for some of these and in some cases coherence issues due to complexities of x-overs and blending (ex. UM3X).
Keep up your searching. I know you've used some of the better products out there. Maybe you need to step to some robust EQ to get one of your earphones to a state you seek. I too still keep looking for the next thing that will outdo the last. I've been fond of a number of products I've used and value several of them in various ways. I'm sure you're like me still looking for that everything product. Out of the slew of IEMs I've used, I felt the RE252 came the closest. I still prefer my OK1 bud a little more though, but it's a product that needs a powerful amp and EQing to really shine. The RE252 doesn't. I've got a DBA-02 on the way, so I'll find out how that stacks up. It'd be fun to at least demo a SM3 at some point. Plus I'm still waiting on the Sleek SA7 which should kick some butt. I'm a big fan of the SA6 which is a pretty killer single BA with an EQed low end. There's always more and more new stuff on the horizon.
