I respectfully have to disagree (sort of) with markd51’s assessment of the benefits of using an SDS, or similar, motor controller. While it may be true that a tt motor controller may not increase the quantity of bass it will certainly increase the quality of the bass response.
The benefits of “addressing speed issues” in turntables go far beyond the correction of obvious pitch instability. Think of this the same way we think about harmonic distortion. Gross harmonic distortion is obvious and is perceived as such. In the case of speed instability (distortion) we hear the dreaded waver (gross) in the decay of notes particularly piano, for instance. However, reduction of speed instability to levels below this point is hugely beneficial. In my experience, the benefits are increases in the pitch definition of bass instruments which results in obvious gains in their musical impact resulting in gains in the overall drive (groove factor) of the music. Improved pitch stability at these fine levels also results in improved overall clarity and imaging. Why wouldn’t it? Pitch waver, as usually perceived, is obvious. At very low levels it is not perceived as such, but as a blurring of overall definition and clarity. In the realm of dynamics this blurring results in a reduction in the musical excitement of a performance which is one of the OP’s complaints about the sound he is hearing.