Random big-box store fluorescent tubes give better light than random big-box store LED "fluorescent tube replacements". That's because of recent technology leaps in real fluorescent.
Brightening up your real fluorescents
But I don't see a way to do this without changing the ballast. Perhaps you could check your ballast and see if it can support more tubes than you have, and hork in another pair of tombstones.
Ballast Factor.
Your first and least powerful option is increasing Ballast Factor. Ballast factor is how hard it drives the tubes, and ranges from 70-130% typically, most being 0.9 through 1.0. This is chosen as an aesthetic choice by the lighting designer. An 80% ballast factor drives an F32T8 at 80% (25.6W instead of 32W).
So check your ballasts for their BF (it'll be stated on the label or the spec sheet). If you don't like the number, you can swap ballasts for one with a higher BF.
I would point out that since ballast factors exist as high as 130% or more, obviously driving the tubes that hard is viable.
T5 tubes.
If cost is no object, the expensive T5 tube offers somewhat higher brightness. Normally it takes a different fixture because tube lengths are different, but for a few lengths, you can get offset tombstones that make up the inch or so difference. Or should I say 25 mm or so; T5 is metric.
High-output tubes.
Many sizes offer "High-Output" tubes. But they use a different ballast, and also a different socket, so you must change the tombstones to "HO" types. Some HO sockets won't fit in space-confined fixtures like a 4' troffer, but you can handle that with T5.
LED replacement "tubes".
The market is flooded with cheap LED replacement "tubes". Most of them output fewer total lumens than a real fluorescent. (though in fairness, if they're better aimed, that may be a net win; none wasted heating up a reflector). If you shop carefully you might find LEDs that give significantly more practical lumens than Normal Output tubes.
However, don't be fooled by apparent brightness caused by the blinding tiny-spot light sources, bluish color temperature or disturbing, weird color mix (CRI) of many. Don't look at the tubes, look at the floor the tubes are lighting up.
DC LED strips + power supply.
At least one company (GE) makes a 12V and 24V DC power supply for LEDs that fits in the standard form-factor of a fluorescent ballast. I have used that to convert troffers to LED strips. Cut the LED strips down to whatever length works, and install as many as you please to whatever brightness you want. It works best in fixtures which have a diffuser, because direct LED strips can be pretty glaring.
These can even be put *in between* the fluorescent tubes. Most 4-lamp fixtures have space for 2 ballasts; make one of them a 4-lamp fluorescent ballast, and the other the LED power supply.
