on bass i 've been using a br600 which has some nice useful patches, right out of box
Yes, the BR-600 is probably a better choice than the Micro BR for serious bass players. Not that you can't get a half decent bass sound with a Micro BR. But all of its amp models are based on guitar amps (Fender Twin Reverb, VOX AC-30TB, Marshall Plexi, etc.), whereas the BR-600 includes two amp models based on classic bass amps (Acoustic solid-state and Ampeg tube). The BR-600 also has a defretter effect (to simulate the sound of a fretless bass), and an octave effect. And there are 14 preset patches for bass guitar that use these effects and bass amp models. So, right out of the box (as you said), there are lots of great effects for bass players to use. And they can tweak all the settings in the bass amp models, defretter, octave, enhancer, compressors, etc. to get an even better bass sound.
For those bass players who've already got a Micro BR and aren't ready to upgrade, I'd suggest looking at the settings of the three "bass" patches included in the Micro BR's presets and tweaking the settings to suit your needs.
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Patch: GT78:Bass Cln
Song: C-ACFG1
Folder: SONG0006.BR0
Algorithm: VOCAL MULTI
Compressor: On
Sustain: 5
Attack: 60
Level: 100
Enhancer: Off
4-band Equalizer:On
LowGain: 0 dB
LowMidGain: -2 dB
LowMidFreq: 160 Hz
LowMidQ: 2
HiMidGain: +3 dB
HiMidFreq: 1.0 kHz
HiMidQ: 2
HiGain: 0 dB
Level: +1 dB
Noise Suppressor: Off
Delay: Off
==============================
Patch: GT79:BassCrnc
Song: C-ACFG1
Folder: SONG0006.BR0
Algorithm: COSM GTR AMP
Preamp: CRUNCH
Volume: 40
Bass: 60
Middle: 50
Treble: 40
Presence: 0
Master: 80
Bright: Off
Speaker Sim: Off
Noise Suppressor: On
Threshold: 10
Release: 30
FX: Compressor
Sustain: 5
Attack: 100
Level: 80
Delay: Off
==============================
Patch: GT80:BassSolo
Song: C-ACFG1
Folder: SONG0006.BR0
Algorithm: COSM GTR AMP
Preamp: Off
Speaker Sim: Off
Noise Suppressor: On
Threshold: 10
Release: 30
FX: Chorus
Rate: 35
Depth: 50
Effect Level: 90
Delay: Single
Delay Time: 500 ms
Feedback: 20
Effect Level: 27
==============================
Notice that GT78:Bass Cln actually uses an algorithm from the MIC patch bank (VOCAL MULTI), so there's no amp modeling at all. And GT80:BassSolo uses a GTR bank algorithm but turns the amp modeling off. Only the GT79:BassCrnc patch makes use of guitar amp modeling to add a bit of distortion ("crunch").