What will a preamp do for me?

The actual answer is in the listening. No theory is going to replace actually LISTENING to whatever gizmo or notion you have hooked up. Sure you can argue theories all day.. But the answer is in only one place. YOUR EARS, listening to whatever.
I own several preamps. and regularly use more than one at the same time. So I can say I am ANTI-(add whatever fancies you). yeah they become ’tone controls’. Why would I care what somebody else calls it? the sound is all I care about. Right now I have my $40 used disc changer to my $7000 SACD player as DAC, to the Threshold FET10 line level, to the Conrad Johnson ACT2, to the Bryston BP-26 (which has the only XLR line out to the amp) SOUNDS great. I can leave off the Conrad Johnson and lose little, but it just has that last extra bit of greatness WITH it in.(probably second harmonics added)

Short answer, yes I am daisy chaining three preamps at this very moment.Debussy in the five disc changer I paid $40 for on eBay$125 Toslink (Lifatech) from that to my $7,000 fancy DAC (or SACD player, your choice but you ask why not just use the spinner in the $7K wonder??? ahhh lazy, five beats one for lazy) The Balanced out goes direct to Bryston BP-26 when I want that. the single ended to the recently bought Threshold FET10 linestage, and from there it can either go to the BP-26, or to my Conrad Johnson ACT2 which in turn feeds another input to the BP-26.                                          
So at the flip of the selector on the Bryston I can hear direct via XLR (Cable is Cardas Parsec) from Marantz SA-10, single ended )able is Kimber KS1016) SA-10 from the Threshold, or single ended SA-10 from the Conrad Johnson via the Threshold(with a Cardas Parsec TO the Conrad Johnson, and a Kimber KS1016 to the BP-26). Direct XLR is the tightest, clearest, alas slightly boring. From the Threshold, more body, more bass and alas, slightly ’dry’(but actually real good). From the Conrad Johnson, ahah! That old second hamonic kicks in... Just right. (everything else stays on 24/7. Sadly the Conrad Johnson ACT2 is blast furnace hot for a preamp gizmo, and only on when used, and yes that is it’s normal state. Curiously the Threshold is stone cold when left on/in use, so is it’s power supply)The difference between choices one two and three is very very tiny. But hey my ears, my money, my choice.
And I LIKE preamps... I own several more... An Audio Research Sp-15 an Audio Illusions Modulus 3A, and a Onkyo Grand Integra P308. Plus the not working VAC Standard in the closet.

Other ways to get better highs are conditioning and aftermarket power cords. Also adding just a volume control like a Goldpoint level controls are very good. Then you would no longer be attenuating the the signal shortening bit length. The Gold Point attenuator could be right at the amp. so the cable to the amp would be a high level signal... (the Goldpoint could be used set to a certain level, near the digital max. then you still could attenuate the digital as now, but it would be only slightly done, and less damaging to the word length.Or, the Gold Point could be right next to you seating position... for ease of adjustment.