There's a series of reviews being done right now on the Darko Audio website. The amps reviewed thus far seem to meet your criteria.
Headphones have been an odd rabbit hole for me. For a while, I was listening to them a lot and bought a few different ones as well as a few amps. They are cool in a lot of ways because it's like being able to swap out speakers and there's no room to deal with and even the upper tier headphones don't cost as much as many speakers do.
But... you have to wear them, and they can be uncomfortable, and you have cables, and... blah blah blah.
The main thing that happened with me is that I kept improving my systems to the point that my speakers sounded as good or better than the headphones and headphones don't really have a "soundstage" or image like speakers do. I still listen to them on a regular basis, but not as much and I have way more stuff than I need.
First, let me say I am brand new to the whole audiophile world. I've owned the HifiMan 400i's for a long time and have always simply used them with the jack on my iPhone or computer. But recently, for some reason (lockdown fever?), the HiFi bug has got me under its spell and I've been buying stuff like crazy. Most of it isn't here yet and even if it was I'm not sure if my ears are keen enough to be a real judge. Having said that, and being fully prepared to get flamed as an ignorant newbie, here are some thoughts that might be a little out of the box.
1) Just got the HifiMan Ananda's yesterday and they blow the 400i's out of the water. I wish I had the Sundara's to compare the two.
2) The FiiO BTR5 headphone DAC/Amp/Bluetooth receiver makes quite a bit of difference streaming bluetooth from an iPhone vs playing from an iPhone jack. That difference is even larger if you use the USB DAC with a PC. I haven't been able to make the DAC work with the iPhone or an old Kindle Fire 8. Buying a Fire 10 HD to see if that works. Nice purchase for $100. I've been using Amazon Music HD as my source.
3) Just for yucks I bought the Douk P1 for $100. It came today. That little device actually checks all your boxes. Bluetooth, USB, and RCA inputs and headphone and RCA outputs.
So far all I can say is that all the functions work and it sounds at least OK, but there is a low level of hiss in quiet passages like Perfume Genius's Moonbend. Not noticeable unless the music is dead quiet. It could be that goes away with burn in or a better DC power supply. The stock tubes aren't supposed to be the best so I bought two others that I get to play with over time. Honestly, just for entertainment value testing the tubes it might be worth the $100. I haven't done a comparison yet with the BTR5.
Probably you are better off sticking with the Schitt Vali or similar and paying more but I like playing with new toys as much as listening to the highest quality music.
Since you mentioned Aune, I suggest you look into the Little Dot amps and see if any of them is a worthy candidate. They offer tube, hybrid and SS headphone amps. Their products are well-reviewed.
About 3 years ago, I got a Little Dot MKIII tube amp for my HD600. It sounds decent, responds to tube rolling and is definitely a good value. I still have it.
If you want something with the same extremely high SINAD performance but with balanced headphone jack as well as balanced inputs here you go:
S.M.S.L SP200 Headphone Amp THX AAA 888 Technology XLR RCA Balanced Headphone Amplifier https://www.amazon.com/dp/B07ZJ6S1DX/ref=cm_sw_r_cp_api_i_bbbnFbA83BZ01
I looked at the Aune X7. At 11 ohms output it is only suitable for high impedance cans.
The Drop is not available for at least 8 months.
They are suspect anyway. When the sold Grados, the Grado company said they didn’t front them any.
Where the hell did they get ’em?
Much stuff NO warranty.
The S.M.S.L. amp I have doesn't sound as good as either Schiit I am using.
Check into the Valhalla model. Reasonably Priced. Feature Packed.
Description, Spec, And FAQ all contain useful information.
OTL, Pre Output, Rollable, and adjustable Gain.
Separates / Component Type Use.
Schiit says: "Valhalla 2 is exceptionally accurate, neutral, and resolving, without being strident or etched. "
My V2 sounds as good as the Mjolnir. A little more expense ($350 new).
Following on the Darko reviews I’ll suggest the DROP + THX AAA™ 789 Linear Amp. Yeah, it’s $299, but you said “maybe a little more” and what’s 100 bucks among audiophiles? The review puts this unit in another league from the very good Asgard 3, so for only $100 more I know what I’d do. And given its ability to scale with better gear it covers you more for when you upgrade your ‘phones. Just my $0.02 FWIW, and best of luck in your search.
Just an update.
I did find a package deal that was an excellent price so went with it.
Beyerdynamic A20 head amp matched with Beyerdynamic T90 cans, both designed to work together and the cans are 250ohm.
First play was last night and a world of difference from my old setup.
A lot clearer and pinpoint imaging, bass although maybe not quite as deep was much clearer and tunefull whereas old rig was more thud, thud on the bass.
For the money I am very happy, a hair more than $200 for the head amp if I split the prices up but way less than $300 so I got what I was looking for.
Many thanks for all the kind suggestions along the way!
Really grown to like and appreciate the strengths of the Beyer combo.
While bass is not ear pounding, it is tight and tuneful and plentiful.
Imaging is glorious and mids and highs are sharp and very well detailed.
Obviously still lower rung stuff compared to what some people have for headphone setups but quite a way elevated from my previous setup.
Yeah, me too. I know that the OP has made his purchase, but for those looking for ideas, The Schiit Asgard 3 is a great option for not a crazy amount of money. I've got a few pairs of headphones, including the HIfiman Drop 5XX, and the Asgard allows such a gorgeous sound stage, wide and with distinct instrument placement.
I had the Schiit Vali 2 before and it's a great option as well, but the Asgard 3 allows more detail, and doesn't have the huge wall wart. In fact, I bought the Asgard 2 on clearance because the Asgard 3 was so impressive. The 2 is warmer sounding which gives a different option, depending on my mood.