LinuxQuestions.org - X fonts get ugly with proper video drivers

I am running Debian Jessie 8.7 on a Lenovo ThinkPad E555 (AMD Kaveri Radeon R4 graphics, 1002:130b).

When neither the free "radeon" nor the proprietary "fglrx" module is loaded, text in X is beautiful - all letters are a nice readable solid black.

When either the free "radeon" or the proprietary "fglrx" module is loaded, letters get ugly and harder to read. For example, in the top bar of the letter "T", some pixels will be black and others will be more gray. Black letters and white letters are both affected - instead of being solid black or solid white parts of them are greyer/fainter.

Exactly the same thing happens with either the "radeon" module or the "fglrx" module, right down to identical patterns in the top of the letter "T", so I'm not sure either module is to blame -- perhaps X has some feature that has a side effect of making letters look ugly, and upon detecting the presence of any module capable of supporting that feature it turns it on?

Any ideas what might be happening here or how to troubleshoot it/stop it?

Edit (additional notes):
1. The actual font is not changing (my subject line used the wrong word), there's just subtle differences in the way the letters display.
2. Resolutions are the same in all cases, and manipulating DPI with xrandr also does not seem to have any effect.
3. I have messed around with using ~/.fonts.conf to turn on/off hinting (made no difference) and anti-aliasing (made letters uneven and blocky with it off) - neither of these seems to be able to replicate the nice solid-colored letters I had when neither the "radeon" nor "fglrx" driver was loaded.
4. It may or may not just be text that is affected. It feels like pictures are subtly different too but it's hard to nail down if there's really anything different there or not.