+1 to the above advice on lowering input level. My interface still passes signal using mic input (I have hot humbuckers), so I set mine to line level and use a slightly higher input level; but still on TH3 I never hit green and the TH3 output is rather high.

If I ever put TH3 in the green, it will distort both TH3 and require my interface input to clip to get there.

Bottom line, be wary about the input meters in TH2/3. Let common sense guide you.

Check this out:
https://www.guitarplayer.com/miscellaneous/-seven-steps-to-amp-sim-goodness
 
I use a DI box, target -10 peaks on the input and use a HP/LP pair in front of sims. 

Hi all, 
 
It seems I've largely solved the problem with the purchase of this: https://www.amazon.co.uk/gp/product/B0036FEKMK/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
 
It seems that despite my Tascam interface offering instrument and line input options, it wasn't enough to get a decent sound. I simply plugged the Zbox into my interface and then my guitar into the Hi-Z port on the Zbox. The result - an absolutely vast improvement. The circuitry seems to do something to the sound to level and smooth everything out creating a much more pleasing overdriven sound. Didn't cost a fortune either.  
 
Paul White, Editor in Chief of Sound on Sound is a guy who's opinion I respect. I found this article: https://www.soundonsound.com/reviews/motu-zboxdi-impedance-corrector plus it's mentioned in an earlier post referencing Craig Anderton's tips for improving the sound of amp sims. 
 
Works for me. Thanks again for all your help. 
 
Dave 
 
 

∞

For gain staging, remember the gain knob on the track. This doesn't risk clipping of the recorded track but can help with the vibe.

mister happy
It seems noteworthy that you have solved your concern by purchasing a dedicated 1MOhm buffer stage. 
 
Did anyone ever ask if you have flipped the switch on your Tascam 4x4 on "IN 1" or "IN 2" from "MIC/LINE" from the 2.2kOhm LINE input setting to the 1MOhm "INST" input setting?
 
If not, give it a try and see if this replicates what the ZBox provides.
 

 
^^^^This.

I have the 4x4's big brother, the 16x08. Even with passive pickups (that is, ALL of my guitars and basses ), I have to set the INST/LINE inputs to LINE.
 
Concerning the Input/Computer knob:
 
Since you are trying to listen to the output of TH3, you need to set the knob 100% to Computer. Anything else will mix some of your input (guitar) with the output coming from TH3. You don't want that.
 
Think of it as a Wet/Dry knob. Dry is your guitar input. Wet is the TH3 output. You want 100% Wet.
 
And hopefully, you're using ASIO and not the Tascam Mixing Driver (if available for the 4x4; it installs by default along with the ASIO driver for the 16x08).

If you're using Sonar, first in the signal chain should be the tube sim, I use this to level the signal and add some 2nd level harmonic distortion. If you're after a crunchy over driven sound perhaps try 1 instead of 2 if you're after a really 80s transistor type over driven sound. I tend to use 2 because it's softer on the highs. Just listen when you dial in the distortion. I then tend to use a low shelf filter - normally the one supplied with GR4...just till off a gentle amount because you will roll off more. I tend to then feed the signal straight into an amp sim. For over driven transistor type crunch you can't go past a marshal sim, if it's softer then a hi watt. You could alternatively add an overdrive pedal into the signal chain before the amp sim. What most people don't quite understand is, that to achieve an album type sound from a guitar sim you then need to mix it properly. After the amp sim I will then use an EQ and compressor. I use my ears to decide which I want first. Also, I use Wave's analouge emulation plugs. So for me, there are some great options that really add to the sound. For example, Kramer's Pye stuff is excellent for rock guitar. Eddie Kramer used a lot of these EQs and compressors on Hendrix's music and also Zeppelin. If you're after a pop type guitar sound you can't go past the TG12345 emulator - The Beatles and Pink Floyd. The Schepp Neve EQ is great too and finally any of the SSL stuff is great on guitars particularly on 80s style over driven guitar. Most of these plugs can be picked up for 30 bucks a pop now too...sweet.

Ben

Let me recommend my preferred amp sims:
Bias
Mercurial
That's it!

Check out YouTube demos and you'll be amazed at how realistic and high quality they sound.

I'd see if I could get a refund, if you're not satisfied with the guitar tones. I think Overloud makes some amazing EQs, but I think there are better options for guitar tones.