Well there are some plus and minuses with the WS55 vs the HTF600. The WS55 are more portable sized meaning they aren't full sized like the HTF600 and depending on your ear size these can be either on ear or over the ear. Mostly on them so the comfort level is not quite the HTF600. These do have an initial clamp but loosens up quickly and fits snug. These are more for traveling and being out doors and such and have a really strong build and above average isolation as they fit snug onto your head. No sound leakage. Sound wise these are great sounding portable headphones but are they really an upgrade to the HTF600 in sound.? In some areas yes the mids,Highs are a bit more accurate as well but overall the sound on these would be more of a side grade than a true upgrade in sound vs the HTF600..These are worth the $80 or so to own them for sure but if you are budget minded listener there are some interesting choices now..
Icipio NX-100 looks like a rebrand of the Brainwaves HM3. Both are supposed to sound good for the cash.
$30-$50 I would save up some cash and get yourself a nice upgrade later on. A nice Denon or a Beyer if your looking more for a full sized can upgrade to the HTF600.. But if you want to have a smaller isolating well built headphone on the go the WS55 are fantastic.