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Hi,
First, I finally placed order of M22ti, Qs4, VP-100. Now to the sub, I'm interested in HSU VTF-2 or STF-2 since they're in my range and based on suggestion from posters here. I'd like to express my thanks to you who has provided me great info.
I keep reading in hsu website, but I still don't get the point in different of them. Can someone please explain it based on your experience?
The difference between the two is that you can tune the VTF-2 for maximum output (louder) or max extension (lower bass)by flipping a switch and plugging one of its two ports with the provided plug. The STF-2 has only one port and is not tuneable, but it sounds exactly the same as a VTF-2 in max extension mode.

Hsu discovered that most people who own the VTF-2 sub keep it in the max extension mode, so you're not really missing much by getting the STF-2 instead of the VTF-2.
Except the opportunity to make a deeper hole in your wallet
I own an STF-2, but have never heard a VTF-2. IF (and that's an "if" in capitol letters) I understand things correctly, the "VT" in VTF-2 stands for "variable tuning." The VTF-2 has 2 ports in the back, and comes with a plug to block one of the ports. Having two ports allows you to choose between:

"Maximum Volume" -- With both ports open the sub can play as loud as it is capable of playing, but only reproduces bass frequencies down to 32 Hz.(It will reproduce some frequencies below 32 Hz., but at 32 Hz. the volume starts to "roll off," which means gradually reduce in volume.

or:

"Maximum Extension" (one port blocked - the sub doesn't play as loud, but reproduces frequencies down to the lowest point it can which is 25 Hz. (Again, it will reproduce frequencies lower than 25 Hz, but with gradually reduced volume as the frequency goes lower.

So, you can choose between having the sub play really loud, but not so low; or not quite as loud, but lower.

The STF-2 is comparable to having a VTF-2 permanently set in to "maximum extension." It has only one port, so you can't tune it. I don't listen at incredibly loud volume, so I didn't feel I needed the "variable" option. I did however, want those lower frequencies, so I got the STF-2. As I said, I've never heard a VTF-2, but, Trust me! The STF-2 plays PLENTY loud enough. If you want the variable feature, you pay $100 dollars more for it.

Hope this helps.
So, STF-2 is probably what i need since i don't play it loud at the maximum volume , but I want the sub to provide full low frequency to compliment M22ti as i listen to music most of the time....

thanks.
Just to make it a little more clear...max output mode has about 2dB or so more output capability than max extension mode.

Also, the tuning is more than just blocking ports, there is also tuning switch in the back of the amp that actually EQ's it for each mode.
précisément

or, in plain english, exactamundo.
To maybe make it easier for you, I have the VTF2. I use it in max extension mode and NEVER touch it. I gave up trying to "tune" everything perfectly. Now I spend time enjoying my music and movies. If you don't claim to be a audiophile perfectionist, get the STF-2 and don't regret it. You'll be happy for sure.
curtis,
Can you please explain a little more? I'm not quite familiar with this.

thanks.
Thanks Curtis,

Audiobob, just in case you didn't know, Curtis is (one of) our resident expert(s) on HSU subs.

And Curis, do you know what the roll off curves are for the VTF-2 and STF-2?
audiofan,

Given a certain enclosure size and ports, you can tune a sub a sub in different ways....deeper extension or higher output. If you tune a sub to hit lower frequencies, you give up output at the higher ones....and vice versa.

Ajax...nope...I do not know the rolloffs. I do believe the rolloffs are natural below the tuning point as opposed to boosted...although I am not sure.

I have a VTF-3 and leave it in max extension. I also got a used STF-2 for use down the road somewhere else....and other than not getting quite as low than the VTF-3....it gives up nothing in my room.
Thanks Curtis.
Thanks curtis,
So, in my case, is VTF-2 is better ? How about STF-2? I look at the pic of the back of STF-2 and VTF-2 in hsu website and I can't tell the difference. There is a litte switch called bass extension. Is this switch for changing between mode in bass extension and maximum output?

If you just want the lower extension, and your room is not large, the STF-2 should be fine.

Yup...that is the switch.
i use the stf-2 in a room that is ~ 14x15ft with vaulted ceiling & an opening to the rest of the downstairs. it blows my older paradigm pdr-12 out of the water with respect to sq and spl.
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