I started learning mixing 6 years ago and never used a sub. it might sound weird to say it but having a sub is more than hearing the low end , it's about hearing the whole true image from bottom to top and you'll mix your high hats right. I used a of 8351 for one year without a sub , it sounded amazing without it but I couldn't apply a corrective EQ the way I really or truly wanted , now I've changed my work flow and the way I approach things and I also can hear what plugins do. also, the dynamics are amazing you'll the difference between compressors. It's worth it to a GLM kit with it for calibration and through the software I can turn off the sub sometime just to hear how everything sounds without it.
If you're considering the upgrade it's totally recommended