None of this matters for sound quality. What's important is that your DAC/amp has been measured here on ASR and that it measured well and has enough power to drive your headphones to the volume that you require. If the Qudelix 5K, which you already own, can drive your headphones you own & will own to a loud enough level then there's no point from a sound quality point of view in buying another DAC/amp. The Qudelix 5K also offers parametric EQ which is exceedingly useful.
Disagreed.
Firstly, Measurments gave you only basic idea if device is good or not overall technically. For example SMS HO200 amp measured better that yamaha pc2002, but sounded worse in all aspects in the same setup. So, its false measurments/lie, mayby device fault, but cant say it was that exact case.
Secondly K9(pro) is great device as EM5 in terms of sound quality by overall people reviews same level of quality. Before EM5 I had topping d70s, so they are the same except topping a tiny bit more accurate/detailed at high spectrum, but tonally not that pleasant as EM5. EM was having a bit more bass (a tiny bit more/"boomy effect"), it was better for my setup.
Usually K9 pro used selling for 600$ or so. For 330$ K9 is very good deal and if you wont like it, you could sell it easily.
About measurments again, for example K371 is very close to harman curve, better than most Dan Clarks, does it mean it will sound better? No. If you will ask "what better" ask 10+ people/read reviews and make overall rate. That would be more "statistically accurate" decision.
If something play loud or even have good SINAD it doesnt mean it will please you at all. Allways look at quality through listening and overall people experiences and comparsions. The more the better.
Thirdly, I have not very costly setup. I would say medicore+, but any EQ through Roon, APO, passive preamps or just any volume knob will degrade sound quality.
Iam really surprised that people with years of music experince and so on even advise EQ. (btw havent tested DSP plugins, but if physical volume degrades sound, why should try.)
Each amp has its own characteristics/"sound character". So, you need to combine DAC/Amp/Headphone strong and weak points to compensate each other.
For example Some vintage SONY ES770/333 or their top version 555 will make sound very dense, soft and iridescent, but with distinct details at high. You wont find it in graphs I guess. Also you wont find anything similar like this series almost certain. Its unique character.
Arya Stealth, Edition XS, HD650 (if you dont mind sound will have some veil), but they are cheapest at used market. r70x if you dont like HD600/650 "veil" and want sound go straight in your head, without any "space" between you and music.
Good amps of top line where allways very costly. Thats why vintage is good cohoice if in good visual/internal condition. They will play as modern ones in terms of quality. Difference is too low, except price will be 2-10x more.
All modern amps Ive heard where poor. So would not recommend any, except cheap price range tier.