I tried a couple filter configurations with the 13700, and I could not get them to work right, but I could plug in a 13600 and they would sweep and filter beautifully... I was reading on some site that the 13600 can sweep easily 6 octaves, but the 13700 struggles to sweep 3-4 octaves. Is this true, or just bad design? I'm still learning (and read an article you linked about OTA's about 3 or 4 times). And, why such a difference?
The only difference (at least that National Semi admits) between the LM13600 and LM13700 is that the bias current for the output buffer on the LM13600 is modulated by the bias current to keep it working best for the operating point of the OTA, and on the LM13700 it's constant. The constant version is better for low control signal feedthrough in audio applications. There is NO difference in the OTA, which is what does the sweep in the filters.
I'm sure you did read that somewhere. However, the internet has made everyone an expert as long as they say "I'm an expert" three times and click their ruby slippers together. I'd be willing to bet that person is telling the truth - for the very limited test they ran. Things like "um, how many of those did you test, and does every LM13600 beat every LM13700, or did you just get a bad one?" and "was this a sub in the same circuit board (and circuit, and parts...) or did you build two different circuits/boards and test them side by side, or test new designs sequentially?" and "was there a mistake in the LM13700 circuit? or the LM13600 circuit?"
Finding consistency takes more than a few test circuits.