I had both the IIIa and the 3.6. The 3.6 is clearly better. Better Bass, Better integration of ribbon and panells and smother all around.
Now the price diffecrnce was substantial. I had a full refurb pr of IIIa for $1350 vs $2750 for used 3.6's.
It was worth it for me.
I would not buy used IIIa's unless they had a complete pannel rebuild. Ribbons are cheap ($100) pannels are $300 to $400 each plus shipping.