I can’t use an outdoor antenna. A powered antenna doesn’t work too well, either. So, it’s just a standard, 75 ohm indoor antenna
There are a variety of powered antennas. Which ones have you tried? When you say a "standard" indoor antenna, again, what do you have exactly? Most "standard" indoor antennas are dipoles, but they’re 300 ohm. Do have a balun attached to a 300 ohm antenna? If so, you’re getting loss through the balun, so you might get better results ditching the balun and attaching the dipole leads directly to your tuner’s 300 ohm antenna terminals.
In any event, the choice of antenna can be as critical as the choice of tuner ... and maybe more so. It isn’t likely a better tuner will perform substantially better in your present location than what you have now, so I’d look to improve your antenna as best you can.