1. The TyrellN6 is a powerful overall plugin that will give you all varieties of nice analog synth sounds. That means wicked basses too. It also has some nice triggering features that are super fun to play around with and a nice, clean interface for easy tweakage. The download page is in german.
2. Instead of bowing the strings like they usually do in an orchestra, with an upright bass you pluck it the strings in various ways to create more of a rhythm and groove kind of feel for the bass parts. Here are my top recommendations for VST plugins and sample libraries focusing on a great double bass: 5 Best Double Bass VST Sample Libraries.
3. So, VST versions of their latest plugins are unlikely to happen. If they wanted to do so, they would have done it years ago with the release of Transistor Bass and Transient Processor. It didn´t happen, so the roadmap is clear.
4. Free VST downloads: Digital Acoustic (Nylon strings guitar by TED) - Spicy Guitar (Physically modelled acoustic guitar by Keolab) - ABPL II (Precision electric bass by Ample Sound) - Bass Module (Bass by MaxSynths) - Classic.Gtr (Spanish guitar by Quiet Music) - Pling4 (Plucked string by Xoxos) - DSK Guitars Nylon (Classic guitar by DSK.
5. Output is famous for creating unconventional breakthrough plugins such as Exhale, Signal, and Analog Strings. Substance is their new powerful bass engine. It’s engineered to combine heavily processed electric and acoustic basses with brass instruments, massive polysynths, and dirty analog synths, all into one instrument.

1. Free Bass Amp Vst

The Roland TB-303 ! the legendary Transistor Bass synthesizer by Roland.
Created to emulate bass sound and used by nearly nobody until someone saw the light : “hey, listen what happens when i turn these knobs!!” turn up the res to the max ! yeah ! aciiiiiiiiiid
Back to the Chicago House, Acid House, Belgian New Beat, then Acid Techno, Acid Trance and every electronic music genre.

One of the best bass VST plugins you’ll find in the market, Waves Bass Slapper extends a library that comprises everything from legato sounds, dead notes to thumbs, popping, pull-offs and more.

Now let list some free TB303 software emulations :

1. Rebirth RB-338 by Propellerhead (Standalone)

Rebirth RB-303, free ! aciid

Noise suppressor vst free. Information and download : http://www.rebirthmuseum.com/

Rebirth was one of the first TB303 software emulation, in 1997!
And it’s free since 2005.
Rebirth is more than a TB303 emulator ; you have two TB303, and TR909 + TR808 drum machine emulations + onboard effects . The software is old, and it still sounds incredible.

As it’s a standalone application, you have to use rewire for sync with your sequencer. Useful informations about sync etc. can be found here : http://www.rebirthmuseum.com/support/support2.htm

Installing Rebirth 338 on Windows 7 may require more steps, you have to download the old winhelp32 to get Rebirth work on Windows 7. Here’s a blog post with all the link you need :
The rebirth ISO torrent, the Win7 installer and the link to the win32help :
http://sporsmaal2blog.blogspot.fr/2010/02/how-to-install-and-run-propellerhead.html

2. RubberDuck by D-Lusion (Standalone)

Download for free ! => http://www.d-lusion.com/ProductsRubberduck.html

This one is even older than Rebirth 338. The sequencer has been simplified, compared to a real 303, and it’s a great gain of time when programming patterns!
There are also onboard effects : a simple but effective delay, and a distorsion unit for getting this squelchy acid sound.
The RubberDuck software is MIDI sync’able, Of course you can record as wav the output of the synth.

3. Creakbox BadBoyz edition (VST)

Creakbox Bad Boys edition

download : Creakbox Bad Boys Edition

The Creakbox VST by Bioroid development was stopped and gone open source in 2004!
The “bad boys” edition adds an effect section with delay & multi distorsion unit : analog drive, rectifier, bit reducer etc. + different filter modes : hipass, bandpass, bandstop, 18 or24 dB / octave.
The sound is good, but unfortunately the sequencer has some bugs in many VST hosts

acid trance !

https://blog.wavosaur.com/wp-content/uploads/2014/06/creakbox.mp3

4. Venom VB-303 by Antto (VST)

/vst-plugin-sounds-different-after-audio-change.html. More about and download here : http://antonsavov.net/cms/projects/venom-vb-303.html

This one is the newest : it’s a VST plugin made with Synthedit. It sounds very good and should work nice in all 32 bits VST hosts.

Acid house!

https://blog.wavosaur.com/wp-content/uploads/2014/06/venom303.mp3

5. FreeBee (workname Lush) standalone

FreeBee 303

Info and download : archive + dl

The Audiorealism bassline ancestor !
This standalone application sounds very good, you can record as wav

mellow acid !

https://blog.wavosaur.com/wp-content/uploads/2014/06/freebee.mp3

6. survey : do you need a free Creakbox VST 64 bit port ?

Post your comment !

Aciiiiid :
Solar quest – Acid Air Raid

Acrid Abeyance – 303 Delight

Dr. Fernando! – Stomach Substance

Plank – Acid Wars

Hardfloor – Acperience 1

Free Bass Amp Vst

Unknown Structure – Helixcoid