Yeah, you may be ok to use even a 15v wall wart. I have used 9v wallwarts in place of 6v ones before, I don't advise it but if you calculate the power (watts) of the 6v model and find a 9v one with similar power rating then the higher voltage probably won't overpower the device. Like a 6v 500ma is 3watts, if you use a 9v 300ma one that would be 2.7watts it may work ok. For recharging batteries like SLA and nicad slightly higher voltage should work ok because the batteries are more forgiving. For some electronics that reregulate DC input to a certain extent should work too. I have seen 6v devices that use 12v dc inputs, and have e 9v dc input boombox that runs perfectly off a 6v SLA. The trick is to limit power on higher than optimal voltage inputs, and to make sure you have excessive power available when lower than recommended for voltage as wallwarts tend to sag in voltage some under load especially the smaller ones while the larger 1amp etc ones sag a lot less. Devices that use small alkaline batteries at decent drive currents may run fine at lower voltages that don't sag under higher than rated current levels. A 9v 200ma device may run fine off a 6v 500ma source, my boombox would start distorting when I turned the volume way up when the 6v SLA I ran it off of in a power outage was about used up. I had a 6v flat alkaline battery in a tv remote tha cost about $7 to replace that I used 3AAA alkalines soldered in series to replace it that ran fine for about 3/4 the length at about 20% of the cost essentially saving me $5.
