Weak Line-in signal using Xonar DX with discontinued Dell C521

I am digitizing my vinyl records from a record player via a preamp to the line-in jack of a Xonar DX card. The input signal is so muted that that going ahead with the task is impractical. I set Recording Devices -> Line In -> Levels all the way to 100 but the sound in my recording program rarely gets higher than -15 decibels. Fortunately my Sigma Tel onboard audio has been able to deliver very good recorded sound through its line-in. In fact a setting of 15 in the Windows Sound applet will deliver audio that approaches 0 decibels or even exceeds it. To date I have tried four different Xonar DXs in my system and they have all had the same problem. The vendor that sold me the board tested one of them on my computer and determined that the card was defective. I have tried resetting my BIOS and have installed the Xonar DX (after disabling the onboard audio) on a fresh copy of Windows 7 (32-bit). The driver installs properly and playing back music in Media Player works fine but when recording the line-in is always weak. Other things that I have tried are: inserting the Xonar in the PCI-e x 16 slot, testing the PCI-e x 1 slot with a NIC inserted in it and installing the latest chipset driver for my computer. The latter is an update called "nVidia SMBus Driver" and is compatible with Win XP but I read on a forum that it was alright to install it under Windows 7.
The same holds true for the Xonar DX's other recording devices i.e. Mic-in, Stereo Mix and Wave when monitoring them in my recording program. I've tried installing just the driver without "ASUS Audio Center."
Do you know of an audio diagnostics program? I tried Rightmark's "Audio Analyser" but it did not work as advertised.

Win 7 is stretching it in a C521 to start with. As you have tried several Xonar cards they must not be compatible with the C521. I can't access the Dell downloads right now to see what motherboard chipset drivers are needed but you can try reinstalling whatever chipset drivers are available as that may help. Usually the chipset drivers are not OS dependent. Also, make sure you have the SigmaTel disabled in the Setup (F2 to access the setup at boot up).

I do a lot of support on the Dell users forum and I don't recall seeing anyone else that is using the Xonar sound card with a Dell. You may also be better off with a PCI sound card rather than a PCIe sound card.

Yes, I found a second chipset driver named "AMD Processor Driver" on Dell's site. I removed the chipset driver that I had and installed them chronologically, but it was no good. The problem persisted even when I was able to install the Xonar DX's driver for Win XP. I nevertheless enjoyed your comments.