So, you are saying, IF the speaker, or amp, or both, are crap, THEN you will get fizz from close-miking. Fair enough. But what if both amp and speaker are not crap, but you still get fizz? Then what? Regarding the amp, I've tried the DSL15C, the DSL20CR, and the 6100LM. You could potentially find fault with the DSLs, but the 6100LM was at one point Marshall's flagship and the amp Jim Marshall called the best amp Marshall has ever built. Hyperbole aside, the 6100LM is a pro-level amp, no matter how you look at it. Many of the guys over at Marshallforum.com swear by it. On the subject of speakers, my DSL15C and DSL20CR are equipped with the Vintage 30 and G12T-75, respectively, and my MX112 cab has a Vintage 30 in it. In addition, I have also tried the Seventy 80 and the G12E-60 that the DSLs came with stock. Again, you could pick on the stock speakers, but the Vintage 30 and G12T-75 are some of the best standard guitar speakers money can buy. What to make of it now?
Well! What can one say? If you are confident both the amplifier and speaker give a good sound and the microphone is ok there has to be another factor causing the fizz because as others have stated, a '57 on or about the speaker fret is how 90% of guitar amp recordings are made. The technique is also common for PA repro.
Are your monitors or headphones a bit shrill? You have to be doing something 'wrong' IMHO.
Dave.