I notice you turned 32 and 64Hz up, but the rest of the bass down, mids and highs at neutral. For more warmth I'd have expected at least 125Hz and 250Hz to be set higher. I don't know whether your speakers put out much at 32 and 64Hz.
As already mentioned, the SA300 provides USB input, just like your D03. The A50 does not.
There are affordable amps with vacuum tubes in the amplification path, but from the little I have read it is not clear to me that they actually do achieve the 'tube sound' you may or may not be after. As in, if they are push-pull (two tubes in differential mode), they suppress instead of enhance the second harmonics. If you find one that actually does enhance the 2nd harmonic, then my next question would be how adding higher frequencies provides more 'warmth', which usually means less not more higher frequencies. In any case, if you buy such an amp, you are pretty much stuck with the distorted sound (you can't out-correct the added harmonics), whereas a parametric equalizer gives you (almost) infinite freedom for your sound curve (but not distortion).