I haven't heard the passive ones, just the active ones.. I guess they sound a bit like eachother, in that case I think they sound very good, pretty natural sound, maybe a little lack of bass.

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David Laine / Castra / Semantica
Magic Island / Armada / Perceptive
Website | | Soundcloud

...not a lack of bass. Simply put, they don't colour the sound...they just give you the actual representation of what is being input. Then you can gauge what needs to be changed (boosted or cut) in your mix.
I don't have any experience with other speakers, but my Tannoy's are really fantastic for mixing... but you'll need to play back your stuff through your stereo or something so you can hear what it will sound like in the "real" world of Souped up systems and car stereos.

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please review my latest AMBIENT track HERE.

Ive only got a budget of around £150, is there any active ones i could pick up? or would i need to shell out more cash

Im using these (passive tannoy reveal) and i got them for £150 from bandland, a really small music store in Middlesbrough. The seem like a good honest pair of monitors, very flat frequency monitor.