I still think running the modern receiver, amp and speakers thru the existing wiring that previous owner performed the conversion presents many variables that are not necessary. You should revisit the instructions for adding the Alpine amp to the Alpine head unit. That amp was designed to be versatile and flexible with both line level and speaker inputs. Line level is the best combination. Also the amp is a good value and extremely small size which makes it universal. All that for $100 or so is a lot to like but not necessarily the best bang for the buck for a straight forward modern head unit to amp to speaker setup. As mentioned above the only difference in 45 watts and 90 watts is 3db more volume before the amp goes into clipping mode, with that said to increase volume 6db one needs 180 watts. I don't think power is the question here as much as "all speakers need to be in phase with each other". My experience is when someone has hacked into the factory wiring at least 1 or 2 speakers are wired out of phase with the others which equals a shallow un-focused sound and a major lack of bass response. That is why I recommended either all new wiring or let an experienced auto sound tech sort it all out.
Have you tried wiring the alpine speaker outputs from the radio directly to the morels? The built in power is plenty .
eliminate the 445 entirely
There's no way the radio could be driving the speakers through the amp. It's not like power steering, where you can still steer if the power assist fails. If the speakers are connected to the amp output then the amp is driving the speakers.
When you say "very weak" what are you comparing them to? Do you simply mean they are not loud compared to the fronts? If so, you may simply have a balance problem.
At low volumes your ear tends to hear mid range sounds better than bass and treble, so if the new speakers are not producing the same sound pressure they will sound like they are lacking low and high frequencies, but in fact they just aren't as loud as the ones you are comparing them with. (Refer to salesman's trick mentioned above.)