I think maybe you need to see how good is acceptable to you. You said "However, it is adequate for creating CDs that have acceptable volume levels - they're just lower in volume than a commercially created CD."
If you aren't getting the audio from very good condition LPs you can probably get away with artifically amping the sound with a program like Adobe Audition (costs money) or Audacity (free). Audition's interface is much better and personally I think its easier to use but Audacity will do the job.
You are going from an analog source to digital, so you can try to minimize signal loss going to the line in on your sound card as much as possible by using good quality connectors, you already seem to have a good enough sound card.
Sound Forge should also be able to amp the signal... but I've never used it.
Anytime you amp it regardless of whether its software or not you are also amplifying noise, so I'm not sure you are going to gain much by going with a dedicated preamp over just doing it through software.