Yes, a specification like the said OPA134 THD value can be verified. The datasheet briefly tells, how.
The datasheets for good opamps show their distortion.
Not particularly, I think. Possibly the datasheets of dedicated audio OPs. Strictly spoken, THD is a circuit rather than a device parameter.
It can be valid for a specified application circuit only, including specified load and other operation conditions. But there are so much other
parameters that can be claimed essential for a "good" OP. Good for what? must be added obviously. From a particular application perspective,
I can tell you also, that a good OP must have at least 1 GHz GBW or 5V/ns slew rate, or ...
But if you're not targetting to high end professional audio equipment beyond usual CD audio quality, the discussion is a bit off topic in this place.
The original poster has clarified, that he's rather interested in a sweepable VCF of tb-303 type than in a fixed frequency OP based active filter.
Extending the dynamic range of a VCF would be challenging. But it's mostly a problem of finding suitable variable gain or resistance elements, as
long as we stay in the analog domain.