Help me make sense of what I'm hearing

Help me make sense of what I'm hearing and steer me in the direction of the right gear.

This is long winded, so bear with me. A little background, this is my current setup:
CD-quality and hi-res flacs playback via linux MPD->optical "DAC Wow" dac->Adcom GFP-555II preamp->Golden Tube SE-40 amp->Kef Q300 bookshelf

Interconnects are Audioquest Diamondback, speaker cables are the Audioquest Midnight 3 Hyperlitz BI-WIRE.

This setup resolves acoustic and electric guitar/bass, upright bass, drums of kinds incredibly well for rock, jazz, blues. Most notable is the setup's ability to reproduce the punch of an upright bass (ex. Waltz for Debby and Christian McBride on Jimmy Smith's Angel Eyes CD) and electric bass (ex. any The Police albums). By punch I mean the natural volume swell (not compressed) - I could feel the bass in my chest without it sounding artificially flubby.

It only does a decent job on vocals and sounds very harsh on metal (ex. Metallica and Mastodon). The vocals don't sound as engaging as it could be and the metal guitars are so forward in the high mids that it's fatiguing to listen to.

Now to the point, I had chance to swap out only the dac for a Rega Dac and only the preamp for a YS Audio Plus-R tube pre. The Rega was polite in the bass and highs so the punch vanished but the metal guitars weren't fatiguing. The punch couldn't be regain by merely increasing the bass gain on the pre and the instruments were a little less resolving.

Next with DacWow back in place and the pre swapped out for the YS Audio, the instruments resolved well and the vocals were the sweetest I've ever heard - very engaging. The punch was kinda there but lower in volume - I can turn the volume up and hear the punch but the whole program was louder than I'm used to. As for the metal guitars, it was still harsh but more tolerable.

So here's the million dollar question: how do achieve it all (if not most of my goals) of punchy bass, sweet vocals, and resolving-ness without harshness?

Is it a different dac, a hybrid preamp, or cotton ear-plug (to filter out the harshness)? :-D

I brought up hybrid pre assuming that it's the best of both world: uncompressed and fast bass (solid-state) with a tube compression of the mids and highs?

Any help would be appreciated. Thanks.
"So here's the million dollar question: how do achieve it all (if not most of my goals) of punchy bass, sweet vocals, and resolving-ness without harshness?"

Good luck.
In your case, I'd start with a high quality tube preamp known to deliver the goods. In order of increasing preference, I'd recommend:

Transcendent Sound Grounded Grid
Modwright SWL 9.0 SE
Modwright LS-100
TRL Dude

No problems with bass response here, and tube rolling has well understood outcomes with the Modwrights.
"It only does a decent job on vocals and sounds very harsh on metal (ex. Metallica and Mastodon). The vocals don't sound as engaging as it could be and the metal guitars are so forward in the high mids that it's fatiguing to listen to."

Metal is harsh and fatiquing to listen to. On CD, LP or live. Your system just seems to be accurate.
I think along the lines of Wilsynet, but in a 180.
Amps add more tube flavor than preamps. You are describing the sonic character of your 555II preamp as the problem. I would change the preamp to a solid state that is known to have that solid bottom, yet smooth and accurate and let your amp give you the warmth & sweetness that you are used to.
Under $1000 I'd recommend:
Muse 3 Signature
Adcom GFP750
Audio Research LS-9 (if using balanced)
Classe 5
You might find a Bryston BP25 at that price.
Kick these around in your search, good luck and I hope this helps.
You can swap stuff around but it is hard to hit a target unless you can locate it.
Get out and listen to other peoples systems until you find an example of what you are looking for. If you aren't willing to do that you are doomed to ride the merry-go-round.
You're the second person to point out that the 555II as being the weakest link in my setup. Thanks for your list of recommendations.

I'll have to read up on your list.

I've suspected that the 555II is accurate and that what I looking for is colored and not neutral.
But his amplifier is already a warm, sweet KT66 / EL-84 based push pull amplifier. So if it's still a bit harsh or etched, perhaps a tube preamp is in order.

As for the 55II being accurate and neutral, just because it's metallic and fatiguing does not mean it is more or less accurate than gear which is warm or euphonic. More likely it's on the lean and fatiguing side of neutral -- steely and metallic is also a kind of coloration.

The best gear that I've heard has great tone, sweet and extended highs, full of body and density, great low end responses, transparent and neutral, and utterly non-fatiguing. But to get all of these things in the same gear, this costs dearly.

For what it's worth, if you only have need of one input, your source has less than 1000 ohm output impedance and your amplifier has 50K ohm or more input impedance, then an LDR based preamp like the Lightspeed Attenuator or the Warpspeed would offer tremendous value for dollar. I have found these types of preamps incredibly transparent without a hint of fatigue (feel free to essage me privately if you're interested in the Warpspeed).
semi-related. Way back in time, I used to own a H-K Citation V tube amplifier (not exactly a mellow amplifier) with a Adcom 565 preamp. It was a terrible combination driving Snell J3s, sounding bright, fatiguing, and annoying. As already stated, I would look at the preamp first - I'm a big Threshold / Forte fan when it comes to used gear. I'm sure there are endless possibilities, but something with a nice FET circuit will work wonders - or a good tube unit.
Of all the reviews I've read, the Modwright seems to fit most of my criteria of being dynamic, deep, with good midrange, as well as being within my price range and having a remote.

Can anyone comment on whether it does metal decently? Or is it a matter of tube rolling to taste?
I would consider the LDR passive approach previously mentioned, your a
amp should match up well, and an inexpensive experiment - if you only have one source.
I would also consider the LDR approach. While there are a number of kit and finished supplier choices, all with their own advocates, if you have more than one unbalanced source, there are multi-input versions of the Warpspeed LDR. Although single input, only the also Warpspeed has a balanced version. If you only need an unbalanced single source, any of the alternatives should work as long as your equipment matches up with the I/O impedances of your LDR choice; i.e., verify with the supplier.
Probably not what you want to hear but your system will shine with small-scale music, not metal or classical rock. For that you need some horns, especially with a nice little amp. Preferably horns with a fairly linear impedance.

If sources are all digital, consider DAC with volume control into power amp into some horns, without a preamp.

Massive solid state power will give you more output before clipping with tighter bass and more freedom with speaker choices.

Not sure if this would be satisfactory with small-scale music (depends on your choices) but if not consider having multiple components to slip in once in awhile as needed. I prefer having a system built for large-scale stuff and using it for the smaller-scale stuff too but YMMV.