I been doing some work on the weekend on your problem.
I beleive I found an SA amplifier can you can use to upgrade your CATV system from 300 MHz to 625 MHz. I found this upgrade module, built by CABLESERV from Canada. You will only have to install this module in your present housing.
HERE ARE THE SPEC OF THIS MODULE:
http://www.cableserv.com/site/CableServ_3/pdf/datasheets/SA%20DA%20FWD%20UPG.pdf
Also include in attachement my calculation of the loss betwwen your present 300 MHz to the upgrade for 650 MHz.
You might have to install a couple of power supplies, you will need to look at your present end of the line voltage.
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This module has 37 dB gain at 650 MHz and I calculated to upgrade your present system to 650 MHz, you'll require an amplifier with 38.1 dB gain @ 650 MHz.
Since your system seems to only have around 300 sub, I believe this amplifier will do the work.
You can get in contact with me @ (-REDACTED-) .
GOOD LUCK and LET ME KNOW THE RESULTS.
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