A technician stopped by. Frankly, it was the first time a Verizon tech support guy was able to provide meaningful information and really try to help. He replaced the ONT, checked and fixed some coax cables, patiently answered all questions, etc. I really appreciated his help.
Bad news, however, is that picture quality remained pretty much the same...
Verizon insists that nothing changed on their side in terms of TV signal characteristics. In other words, the STB (after upgrading to Quantum) receives absolutely the same signal as before the upgrade. I somewhat resistant to believe in this theory. During the initial Quantum setup, the only thing I did (I do not describe here the Internet part, this is only about the TV) is replaced the old STB with the new one. Did not touch TV picture settings, coax cables, a splitter, etc. But the difference was more than obvious.
Whatever was done after that - new splitters / cable fixes / ONT / etc. - look like a plastic surgery on a 90-years-old bottom: you may or may not see a tiny difference, but it does not affect the essence. "Clouds" on the ice hockey ring (which makes it looking like you're youtubing a low resolution video) are still there. Verizon technician noted by himself that colors are horrible. He mentioned, however, that this must be broadcasting quality issue. I was not convinced. I'm watching the same channel for years and never saw anything like that...
Don't know what to say... Still have time till Apr-5 (30 days after services activation) to get rid of it...