Is your a V1 or a V2 HD9?
The V1 uses a standard MB embroidery machine bobbin size, but a half-moon case. V2 uses a slightly smaller proprietary bobbin size and full bobbin case.
If you have a V2, can you use prewound bobbins in it (like DecoBobs or Magna Glide?)
I have the V2. It does not take prewound. Janome stopped making the V1, they changed the V2 to the larger bobbin. It is not a small bobbin, it holds 1.6 more thread than the Juki 2010. The note below was from a previous post on this machine.
JANOME HD9 V2 was my choice after watching the comparison on YouTube .
Janome has an extra large bobbin. Juki is Lstyle bobbin.
HD9 foot pedal is the extra large one. Juki is designed where you have to have your whole foot on top of the pedal thus you have to hold your foot flat on the pedal putting strain on your foot and calf.
Janome throat space is a little larger.
Hd9 max sewing is 1600 spm, Juki 2010 is 1500 spm
HD9 has a lot more lighting than the Juki2010.
HD9 has a higher foot lift at 12.7mm vs. Juki 9mm.
The biggest selling point for me was the Janome sewing through 4 layers of leather. I make bags, so important to me. Test on YouTube, same leather, thread, and settings, the Juki would not sew, machine jammed, broke thread. Watch "let's talk Janome versus Juki HD machines - Kelley Rao, from the 10:55 minute mark to 13:16 it shows the issue with the leather.
I would say try them, check out all the features from lighting to foot pedal to size of bobbin. Watch the you tube videos. There are two by MA Couture on the HD9.
They are both nice machines. Also, when you choose, ask the dealer for walking foot, extra feet, needle plate, bonus bundle etc.
BTW the industrial sewing machine feet from Wawak are a lot less in price and fit both of these machines.
best wishes in your quest.