I noticed a big difference from adding more volume while still doing two higher intensity rides per week. Maybe it would have happened anyway. But after slacking off last Summer I did more volume in the Fall and by late Spring (that is, now) I'm in clearly better shape. Power across all times above about 3 minutes are up across the board.
For me, "more" was upping from about 6 hours to 9-10 hours. I've had a few 11-12 and a few 7-8 but mostly 9-10. Lots of work at what turns out to be around 60-65% FTP. Also, when I was training more I was getting more sleep. It was more of - when I get lots of sleep I am ready to train more. I don't think it's the opposite - training more means I get more sleep.
Yesterday I did the equivalent of a 12 minute TT (it's for the Navy fitness test) and I set a new PB at about 335W average. So whatever I'm doing is still working for me.