Hello, my name is Rusty. Thanks you for the details, since it won't play from iTunes, that tells me that your audio settings in your iMac aren't correct. Audacity uses the current settings, unlike Pro Tools which makes you select the audio device.
That should get everything working. It sounds like your input is already set to the Tascam, let me know if you have any trouble.
Ok, click here. The ASIO4ALL driver will usually fix that. Since you're unable to move the faders, there seems to be a lack of communication between your Mac and the interface. Let me know the results.
Sorry for the delay, I'm on the West Coast USA. The ASIO driver is what you need to install. Don't put it anywhere, just open the file, and your computer will put it in the correct place. That is usually the solution for all interfaces.
To be clear, you're opening the .dmg file, right?
Are you able to open the USB-ASIO driver?
Ok, and in your Mac audio settings, is the interface selected as your default output device?
Which OS version are you using? With Mac core audio, sometimes the input will work, but not the output, and vice versa. I want to make sure the 1800 is completely compatible.
Ok, that is compatible. Click here. Install the driver for Yosemite, and that should allow you to select the 1800 as your output device.
Hmm, this is not normal... Are you able to hear audio from your computer's headphone output?
It's the core audio that is giving us the problem, Mac works differently than Windows 7. The trouble is getting the interface and Mac to speak the same language, which is what the driver is supposed to do. Something is getting in the way, and I'm not quite sure what. Delete the driver, and install it again. Sometimes other devices can take priority, causing this issue. Reinstalling the driver will sometimes put it out in front so it's first in line when booting.
I'm going to have to open this question back up. I don't know if there will be a solution to the problem, there is a conflict with another device, and those can be impossible to find.