I love Heiller's Rameau on harpsichord for its incredible sense of rhythm and lyricism. When I first found this LP over two decades ago, I listened over and over again, and had these pieces stuck in my head for months. William Christie's performances are quite good, Scott Ross' were a big disappointment (too fast and zero feeling), and various other versions on harpsichord and piano I have heard were ok, but for me none will ever compare to Heiller.
I am sure the instrument he used was not a Pleyel, steel frame affair. The instrument is actually very beautiful sounding and seems historically correct, but the sound quality is far from great.
His Bach seems equally perfect, with all the right tempos, articulation, beautiful registration, etc. Of course, I also love Karl Richter's organ and harpsichord performances, and everyone else on this forum seems to hate and detest Karl Richter, so keep that in mind.

Heiller was also a noted music scholar and composer.