Coming back to the Ab and Db, I believe that the absence of sympathetic resonance for these notes contribute to give them some "thumpiness" with less sustain and brilliance than the A and G neighboring Ab, or the C and D neighboring Db. Same consideration holds for the thumpiness of the low F.
On re- reading this thread's contributions I picked up on this quote for further comment.
The "thumpiness" could be the result of habitually using the weaker pinkie, supported by 3 or not, to play A flat and D flat. If I want the greatest clarity of these notes I will play them with the stronger 2 supported by 1. The extra tightness of the skin at the exit point of the string helps clarity and sustain by not damping the string as much. Try each possibility and hear the difference.
The bottom F is affected by the thickness and weight of the E string, the clearance of the string at the nut and the proximity to the nut. Here you are relying on the strength and toughness of pad (callus?) of the index finger alone to press the string down where it is stiffest (least flexible).
Another reason for "thumpiness" is that the bow contact point or pizzicato plucking point need to vary for best results, note by note, at the same time. You should consider avoiding bowing/plucking on a strong harmonic point above the fingered note because that harmonic will be absent in the sound, perhaps deadening it.
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