Oracle 11g R2 – Unable to Turn Off Automatic Memory Management (AMM)

i am running oracle 11.2.0.4 SE RAC on Red Hat linux 6, the database is using ASM for datafile management

i have been experimenting with turning on AMM and set the following parameters

alter system set pga_aggregate_target='0' scope=spfile sid='*';
alter system set sga_target='0' scope=spfile sid='*';
alter system set memory_max_target='8G' scope=spfile sid='*';
alter system set memory_target='8G' scope=spfile sid='*';

but now i wanted to turn off AMM on the instances and database, but when i make the required changes

alter system set pga_aggregate_target=2G scope=spfile sid='*';
alter system set sga_target=6G scope=spfile sid='*';
alter system set memory_max_target=0 scope=spfile sid='*';
alter system set memory_target=0 scope=spfile sid='*';

i receive this error when starting the instance

SQL> startup pfile='?/dbs/initdev12.ora';
ORA-00843: Parameter not taking MEMORY_MAX_TARGET into account
ORA-00849: SGA_TARGET 6442450944 cannot be set to more than MEMORY_MAX_TARGET 0.
ORA-01078: failure in processing system parameters

if i set the MEMORY_MAX_TARGET back to 8G i can start the database but my parameters now look like this

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
lock_sga                             boolean     FALSE
pre_page_sga                         boolean     FALSE
sga_max_size                         big integer 6G
sga_target                           big integer 6G
SQL> show parameter memory

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
hi_shared_memory_address             integer     0
memory_max_target                    big integer 8208M
memory_target                        big integer 0
shared_memory_address                integer     0
SQL> show parameter pga

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
pga_aggregate_target                 big integer 2G

any ideas what i'm doing wrong?

cheers
dave