I owned a Strategy and the SB300 at the same time. They sound very very different. I’m going to give you my honest personal opinion, with the understanding that these are two great amps and we each hear things differently.
The Strategy is extremely powerful and has a huge low end and an extended top end. I found it very modern and scooped. It has a number of voicings to choose from, and all but one are variations on a mid scoop. The remaining one is “flat”, but still scooped. It sounded to me a lot like the M9, but more scooped. It did not feel or sound like what I hear as “tubey.” There is a lot of eq, but the amp has a character and doesn’t stray too far from that. The preamp distortion sounded terrible to me, splattery and fuzzy.
The SB300 is very tubey, punchy, and fat sounding. It has the traditional Fender tone stack on channel 1, which is beautifully vintage sounding, nice and plump and organic. The other channel has preamp distortion which is decent but not great. Channel 2 also has an active semi parametric mid control, allowing you to get more middy modern tones. It can do a good replica of an SVT, an old Bassman, and a number of other sounds. Extremely versatile, loud, and MUSICAL.
I sold the Strategy a couple of years ago, and I still have the SB300. It’s kind of getting to be too heavy for me these days, but man, what a great amp. Between these two amps, for me the Fender is the winner hands down.
I also owned an SVTII at the same time as these two. The SVTII actually had the best default (“flat”) tone for me, and the most intuitive eq. But it’s HEAVY and not as versatile as the Fender.