The funny thing about the SBL is it always ends up being talked about in the same company as the ATCs, Dutch and Dutch etc. Let's remember the SBL is a 30 year old plus design and can be had for a few hundred quid these days. If you like the Naim sound and can get them set up properly the bang for your buck is hard to match, but do not expect them to do what a newer speaker costing £4K in passive mode and £7K in active mode can do. Woodface had a very nice set of SBLs and went to ATCs, but I think he went from passive to active. SBLs driven actively can produce some very nice results, I once heard a 'cheap' active system using SBLs with a 72 and 2 140s... it had a sound way above the cost of the components.
That said they are a bit fiddly to set up and now Allaes are at rock bottom prices there is an argument to look at the more simplistic and easier to live with speaker, but perceived wisdom is the SBLs are better speakers. Personally I think the Allaes are Naim's best kept speaker secret, but maybe that's just me. My current set cost me £360 and are immaculate, they are currently being driven vvy Teddy Pardo amps and a CD5X and that makes a very nice sound, well for us Naimees at any rate