Sonar X2 - Tascam US-1800

Uh, turn up sonar's track to 0 dB.  See if that don't fix things.  It won't have any effect on the recording, but on output.
 
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This is one of the fishy endings that I find hard to swallow. 
 
First, Level settings and gain are set at the interface, changing a setting within a DAW has no effect on gain or the recording level. Some interfaces my have a DSP control panel/ mixer but the Tascam does not. All input gain is controlled by the hardware, not software. 
How to do this properly- 
The Tascams 2 Instrument inputs 9/10 have both input gain control and a pad switch. 
Pushing the switch in ( Guitar) will result in the input becoming more sensitive (louder) and as an example, a Telecaster on bridge will clip ( RED LIGHT ON ) at about 10'o clock on the gain control. 
With the pad switch out ( Instrument) I will need to turn the gain to around 2'O clock to clip the signal-(see a RED light.)
The RED light should correspond to what you will see in your DAWs meter if set in RECORD mode.
So you obviously do not want a red light or you will hear distortion..the wrong kind.
 
2nd. There are reasons that using that jack to record an electric guitar is a bad idea in the first place.
The Tascam is well known to have pour performance with Round trip Latency. (RTL) You will not be able to use Guitar Rig successfully. There will be around 20 ms of delay from when you play a note to when you hear it while monitoring any real time EFX. The Tascam it self adds 12ms of delay to a none processed signal and adding a VST plug in will increase this further. With my system this is 20ms so therefore unusable.
 
Secondly the 9/10 inputs are not exactly going to give you great tone to begin with. Electric guitar PU's need a pre amp. Preferably a nice tube one designed for guitar. Therefore the tone will be thin. The jacks work fine for acoustic electric guitars that have on board pre amps. Some Basses will sound OK too.  
 
Also hopefully you have figured out the Tascam's Monitoring set up. You need to work with the controls to get a nice loud signal in the headphones. You should be direct monitoring your input signal which you accomplish  by turning the MIX control towards INPUT. I find about half way is a balance of the computer and whatever I'm overdubbing. Then turn up the headphone control or the monitor control depending on what you need. You should hook your Nearfields to the MONITOR jacks in the back not the 1/2 or 3/4 so as to better control. 
The headphones should be pretty loud at this setting, but some headphones are quieter than others. 
 
Lastly, You may need to totally remove the Delta PCI card as that can often cause conflicts with the Tascams ASIO drivers. 

I record my guitar with a mike on my Amp and sometimes through my efxs pedal board set up. I record dry mostly and mess with the efxs afterwards, But some EFXs are driven by the way you play so I sometimes use them live.
One way is to record both a dry and an wet (efxs) tracks. I do this via my Boss Chorus pedal which is first in my pedal chain. It has a wet/dry out. Dry goes directly to the amp/ SM57 XLR into Tascam CH 1 and wet goes to the rest of the pedals and out to the Tascam CH9 input. These are mono tracks and I can mess with the stereo image this way.
 
The Tascam is a well built interface and has a nice green check marks beside all the features I need. Only the driver issue with RTL is an negative issue. All else is good.  I just don't use real time efxs. No biggy. Someday I'll buy a simple 2 ch interface with low RTL as a live rig but the Tascam should serve me well in the home studio for a long time yet. Indispensable if you record real drums.