I'd be surprised if your 4500 watt amp didn't have a gain pot for each channel specifically for this reason, if nothing else they will be on the back of the amp and you'll need a screwdriver to adjust them. But, I've been wrong before. The solution to an amp with no gain control is to add a passive preamp between your DAC and your amp. For passive preamps, look at something like the Schiit Sys if you're running RCA interconnects or the Nobsound Mini if you need XLR connections. I own the Schiit product, two actually, and they give no troubles.
If you're still confused, please list your entire audio chain starting with how the audio leaves your computer and ending with the model of your amplifier. Be specific.
Home Desk ~ MB 3.3 Portable • Questyle CMA400i (ASIO) • Sennheiser HD 660S (balanced) / Audeze EL-8 Closed Back / Fostex TR-X00 Ebony • Teac AI-101DA • Jamo C93 + Dayton Audio SUB-1000
Work Desk ~ MB 3.3 Portable / Tidal • SMSL SU-8 v2 • Nobsound NS-05P • THX AAA 789 • Sennheiser HD 58X (balanced)
OTG ~ FiiO M11 • Audiofly AF180 / B&O H6