Archimago's Musings: MEASUREMENTS: Melody Onix SP3 Mk. II (~2006) Class AB Tube Integrated Amplifier. (And it isn't "diminishing returns" on sound quality, often there are simply "no returns".)
Story of Jadis Electronics : tube power amplifiers/preamplifiers in Villedubert (France, 11)
The Best Amplifiers | Buyers Guide 2021 - Part-Time Audiophile
Amazon.com: Fosi Audio P1 Tube Preamplifier Mini Hi-Fi Stereo Preamp 6K4 Valve Vacuum Pre-amp with Treble Bass Tone Control for Home Theater HiFi System(not for Turntable) : Electronics
Amazon.com: Fosi Audio T20 Bluetooth 5.0 Tube Amplifier Headphone Amp Support aptX HD Stereo Receiver 2 Channel Class D Digital Mini Hi-Fi Power Amp for Home Passive Speakers with 6J4 Vacuum Tubes :
High Fidelity
10 Tube Amplifiers that Are Essentially Works of Art – Robb Report