The biggest issue your'e going to run into is the lack of gain with a dynamic microphone like the SM7. It was designed to be an in your face microphone used by radio announcers. Even if your preamplifier has 70db of gain you may still find it lacking for distant mic'ing. Comparing the dynamic SM7 to a condenser like a 416 is like comparing a sports car to a pickup truck. Both were designed to do what they do extremely well. They're just not what you could consider interchangeable.
As far as a "toob" preamp, unless you spent more than a thousand dollars on it, you probably have what is referred to as a starved plate design. With this type of preamp the heavy lifting is done by a solid state operational amplifier and the "toob"is pretty much a distortion maker. The plate voltage in the "toob" is made intentionally low to cause the waveform to flatten out, theoretically adding pleasant harmonic distortion to the clean signal. This may actually work for VO if your'e careful with your levels, but generally you would be better served to have a quality preamp to begin with. Expensive... yes... But you're probably not shooting with Tamuron primes either.
Lastly.... Before you spend another penny on audio equipment, please contact me. I have thousands and thousands of dollars of recording equipment sitting around here and I don't want to admit to you how much of it was a waste of money.
Respectfully submitted, olduncledino