Amplifiers I've tried the Stealth with, in rough performance order. The source for each amp is at least in the same, or a similar price bracket, if not a higher one.
Schiit Magni Heresy
Can easily hit maximum volume in high gain without the sound getting too loud. Soundstage compressed. Bass control poor.
Audio-gd R2R-11 M2
Pleasant sounding, but lacking in clarity. Some soundstage compression. Better as a DAC to a better amp.
Topping A90D (balanced output)
Soundstage is more open, but very left-to-right, without a sense of front-to-back depth. Instrument and vocal position moderately indistinct.
If paired with a D90 without USB isolation, the sound is "hard" and somewhat unpleasant. Either a non-mains-connected source, or a USB isolator or similar is needed to rectify this.
FiiO M17
Headphone drive is excellent, but can sound a bit flat or plain with the ESS-based DAC.
Chord Hugo 2 + 2go
Better detail and separation, with more of a sense of depth. Nuances in individual notes more apparent. Gain slightly limited, with the volume control heading up to white with classical.
Headamp GS-X mini
A touch warm-sounding. Has a slightly heavy/punchy sound with better depth than the $500 amps. Needs to be used with the balanced output in high gain for optimum performance. Not the widest, deepest or most nuanced, however.
Audio-gd R-27HE (USB, OS8)
Excellent headphone drive. Not as nuanced as the Chord gear, DAC-wise, or the ALO Studio Six amp-wise, though this can be improved considerably with HQPlayer.
ALO Audio Studio Six
More nuanced sound, but without as much bass punch, and not as much raw detail as the TT2. Impressions are very dependant on the selected tubes and warm-up time.
Chord TT2 (with or without MScaler)
Can easily hit -0 dB from the front outputs in high gain, but otherwise headphone drive is excellent. The bass doesn't sound as strong as it does from other amps, but possibly from less from bloating/lack of control. Music nuance and flow is a distinct step up from the Hugo 2. May give a sound that is too "bass light" for some people.