I'm using Guitar Rig 4 for my records and I'm actually happy with my results. I always plug my guitar directly into my USB interface. I'm just wondering if using a preamp would make my songs sound better. I won't know how much I'm missing out until I try

I don't really have any money to spend, so I'm wondering if plugging my guitar into my amp (guitar -> guitar amp clean channel)/bass-> bass amp) and using the line/headphone out ---> USB interface would work. I'm sure it won't sound as good as a real tube preamp, but like I said I have no money so I want to try at least something, but without ruining my stuff. Thanks.
Well we tried doing that to save some money when we started out but the line outs on our amps were terrible. I'm not sure what your situation is but we decided to go get a Digitech multi effects processor. We used that as our preamp and it worked really well but in the end we figured out it was much better to mic your amp. Either that or putting the guitar through the pedal and into an interface. If you really don't have any money to spend then I'd try using the line our on your amps... but if you really want the sound quality to improve I'd look into scraping up some money for an effects pedal or get some microphones so you can mic your amp. It's not incredibly expensive. You can find nice multi effect processors for around 100 to 150 and we got a really good mic set with 2 very good mics for about 75 bucks. So just go to guitar center and ask around. They know pretty much everything from experience so thats where I go. Just tell them your budget and what you need and you'll be doing pretty good.
One thing you could do is use a combination of Guitar Rig and your amp's preamp. What you do is just use the cab sim on Guitar Rig, maybe some modulation fx, and your amp's line out. You could probably get a decent sound out of it.