We all love to upgrade ( I just meant the speakers)
Since audition is difficult, you will have to make a blind purchase. Of course we all have made blind purchase at least once and the outcome was sometimes rewarding and sometimes an expensive lesson. Of course that is part of the AV life.
I will just explain the characteristics of some subwoofers (These are just my perception). Hope you get some clarity.
SVS: Very good for movies and average for music.Thick and heavy sound with lot of bottom end and average midbass. Has a lot of features and impeccable after sale support and warranty but pricy compared to the US price.
RYTHMIK: Very good for music but will be behind the SVS for movies. A bit lean sounding compared to the SVS but has even more bottom end than the SVS and can even reach 12.5hz with authority but the midbass strength varies based on the model. Very musical and very fast and tight but some people may need getting used to the sound of it to like it. Good built quality like the SVS but less feature rich compared to the SVS. The price was good but the recent hike (The price got increased twice in India during 2020 even though the International price had no change) in the indian price has taken it closer to the SVS in terms of price, which is a disadvantage as SVS has already been well established in India and no other brand until now can match the customer support provided by the svs.
REL : IF you are a 100% music guy then this is the sub most of us will recommend you until you can get a pair of Seaton Submersive HPi or a pair of Funk Audio Subs. Very nice mid bass, but very poor bottom end. Very good for music but not recommended for movies. Very fast and clean sounding. Their pricing is also in pair with the international price. Well built and very very beautifyl. Got decent features like High level input. But a remote app would have been great. Will have less placement issue as its lowend isn't that strong.
XTZ: These subs bas a bit of both the worlds. Decent Bottom end but not comparable to SVS or Rythmik. Midbass is also decent. Not so clean as the Rythmik or fast as the REL but a Jack of all trade. Pricing is also good. Recently they have launched Sealed dual opposed 12" 1000W sub, which may be more musical (just my prediction). Have also lauhcned a 15" ported model to compete with the big boys like the SVS and Rythmik.
Velodyne: Good for both movies and music but extremely over priced.
BIC: Good sub for beginners. Good value for what you pay. Will give you a taste of what HiFi subs are all about. Any person without exposure to highend sub will love this sub. Does every thing well for the money.
Hope it helps.