If we were neighbors, you would see an HD7964P or HBU-33 or 44 on my roof.
Your investigation of signal conditions using the DB-4 tells quite a bit about what you can expect in your attic. The numbers suggest that the attic environment is handicapping your antenna by more than 40 dB of noise margin. Though as JC has suggested -- cable, connectors or splitters could be causing trouble... or all of the above. Definitely worth investigating thoroughly.
The
DB-4 specs say it is able to offer 13.7 dBi gain which is about 11.5 dBd. The high performance Winegard HD7698P
specs peak at 14.2 dBd in the UHF band. 3 dB of additional gain is not likely to offer much improvement in your situation. The smaller antennas you mention will offer less UHF performance, making your situation worse.
I can't help but conclude that your attic presents some serious reception challenges. Putting a significantly larger antenna in your attic
may help... but the data you have already gathered tells me you could put the largest available antenna in your attic and find little improvement.
A DB-8 and a YA-1713 joined with a UVSJ would be more manageable in my attic than a single large combo antenna. (But that doesn't change my opening observations. If you try that option, try to use a dealer with a very liberal return policy.)
As others have observed, you are in a location quite hostile to effective operation of an amplifier. And an amplifier is not a substitute for an antenna anymore than a pump is a substitute for a well with clean water at the bottom.
If you can not or will not consider outdoor mounting, you may be limited to little more than what you receive now. I would be interested to know what you receive if you try the DB-4 outside....